Root/fs/mpage.c

1/*
2 * fs/mpage.c
3 *
4 * Copyright (C) 2002, Linus Torvalds.
5 *
6 * Contains functions related to preparing and submitting BIOs which contain
7 * multiple pagecache pages.
8 *
9 * 15May2002 Andrew Morton
10 * Initial version
11 * 27Jun2002 axboe@suse.de
12 * use bio_add_page() to build bio's just the right size
13 */
14
15#include <linux/kernel.h>
16#include <linux/module.h>
17#include <linux/mm.h>
18#include <linux/kdev_t.h>
19#include <linux/gfp.h>
20#include <linux/bio.h>
21#include <linux/fs.h>
22#include <linux/buffer_head.h>
23#include <linux/blkdev.h>
24#include <linux/highmem.h>
25#include <linux/prefetch.h>
26#include <linux/mpage.h>
27#include <linux/writeback.h>
28#include <linux/backing-dev.h>
29#include <linux/pagevec.h>
30#include <linux/cleancache.h>
31
32/*
33 * I/O completion handler for multipage BIOs.
34 *
35 * The mpage code never puts partial pages into a BIO (except for end-of-file).
36 * If a page does not map to a contiguous run of blocks then it simply falls
37 * back to block_read_full_page().
38 *
39 * Why is this? If a page's completion depends on a number of different BIOs
40 * which can complete in any order (or at the same time) then determining the
41 * status of that page is hard. See end_buffer_async_read() for the details.
42 * There is no point in duplicating all that complexity.
43 */
44static void mpage_end_io(struct bio *bio, int err)
45{
46    const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
47    struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;
48
49    do {
50        struct page *page = bvec->bv_page;
51
52        if (--bvec >= bio->bi_io_vec)
53            prefetchw(&bvec->bv_page->flags);
54        if (bio_data_dir(bio) == READ) {
55            if (uptodate) {
56                SetPageUptodate(page);
57            } else {
58                ClearPageUptodate(page);
59                SetPageError(page);
60            }
61            unlock_page(page);
62        } else { /* bio_data_dir(bio) == WRITE */
63            if (!uptodate) {
64                SetPageError(page);
65                if (page->mapping)
66                    set_bit(AS_EIO, &page->mapping->flags);
67            }
68            end_page_writeback(page);
69        }
70    } while (bvec >= bio->bi_io_vec);
71    bio_put(bio);
72}
73
74static struct bio *mpage_bio_submit(int rw, struct bio *bio)
75{
76    bio->bi_end_io = mpage_end_io;
77    submit_bio(rw, bio);
78    return NULL;
79}
80
81static struct bio *
82mpage_alloc(struct block_device *bdev,
83        sector_t first_sector, int nr_vecs,
84        gfp_t gfp_flags)
85{
86    struct bio *bio;
87
88    bio = bio_alloc(gfp_flags, nr_vecs);
89
90    if (bio == NULL && (current->flags & PF_MEMALLOC)) {
91        while (!bio && (nr_vecs /= 2))
92            bio = bio_alloc(gfp_flags, nr_vecs);
93    }
94
95    if (bio) {
96        bio->bi_bdev = bdev;
97        bio->bi_sector = first_sector;
98    }
99    return bio;
100}
101
102/*
103 * support function for mpage_readpages. The fs supplied get_block might
104 * return an up to date buffer. This is used to map that buffer into
105 * the page, which allows readpage to avoid triggering a duplicate call
106 * to get_block.
107 *
108 * The idea is to avoid adding buffers to pages that don't already have
109 * them. So when the buffer is up to date and the page size == block size,
110 * this marks the page up to date instead of adding new buffers.
111 */
112static void
113map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
114{
115    struct inode *inode = page->mapping->host;
116    struct buffer_head *page_bh, *head;
117    int block = 0;
118
119    if (!page_has_buffers(page)) {
120        /*
121         * don't make any buffers if there is only one buffer on
122         * the page and the page just needs to be set up to date
123         */
124        if (inode->i_blkbits == PAGE_CACHE_SHIFT &&
125            buffer_uptodate(bh)) {
126            SetPageUptodate(page);
127            return;
128        }
129        create_empty_buffers(page, 1 << inode->i_blkbits, 0);
130    }
131    head = page_buffers(page);
132    page_bh = head;
133    do {
134        if (block == page_block) {
135            page_bh->b_state = bh->b_state;
136            page_bh->b_bdev = bh->b_bdev;
137            page_bh->b_blocknr = bh->b_blocknr;
138            break;
139        }
140        page_bh = page_bh->b_this_page;
141        block++;
142    } while (page_bh != head);
143}
144
145/*
146 * This is the worker routine which does all the work of mapping the disk
147 * blocks and constructs largest possible bios, submits them for IO if the
148 * blocks are not contiguous on the disk.
149 *
150 * We pass a buffer_head back and forth and use its buffer_mapped() flag to
151 * represent the validity of its disk mapping and to decide when to do the next
152 * get_block() call.
153 */
154static struct bio *
155do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
156        sector_t *last_block_in_bio, struct buffer_head *map_bh,
157        unsigned long *first_logical_block, get_block_t get_block)
158{
159    struct inode *inode = page->mapping->host;
160    const unsigned blkbits = inode->i_blkbits;
161    const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
162    const unsigned blocksize = 1 << blkbits;
163    sector_t block_in_file;
164    sector_t last_block;
165    sector_t last_block_in_file;
166    sector_t blocks[MAX_BUF_PER_PAGE];
167    unsigned page_block;
168    unsigned first_hole = blocks_per_page;
169    struct block_device *bdev = NULL;
170    int length;
171    int fully_mapped = 1;
172    unsigned nblocks;
173    unsigned relative_block;
174
175    if (page_has_buffers(page))
176        goto confused;
177
178    block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
179    last_block = block_in_file + nr_pages * blocks_per_page;
180    last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
181    if (last_block > last_block_in_file)
182        last_block = last_block_in_file;
183    page_block = 0;
184
185    /*
186     * Map blocks using the result from the previous get_blocks call first.
187     */
188    nblocks = map_bh->b_size >> blkbits;
189    if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
190            block_in_file < (*first_logical_block + nblocks)) {
191        unsigned map_offset = block_in_file - *first_logical_block;
192        unsigned last = nblocks - map_offset;
193
194        for (relative_block = 0; ; relative_block++) {
195            if (relative_block == last) {
196                clear_buffer_mapped(map_bh);
197                break;
198            }
199            if (page_block == blocks_per_page)
200                break;
201            blocks[page_block] = map_bh->b_blocknr + map_offset +
202                        relative_block;
203            page_block++;
204            block_in_file++;
205        }
206        bdev = map_bh->b_bdev;
207    }
208
209    /*
210     * Then do more get_blocks calls until we are done with this page.
211     */
212    map_bh->b_page = page;
213    while (page_block < blocks_per_page) {
214        map_bh->b_state = 0;
215        map_bh->b_size = 0;
216
217        if (block_in_file < last_block) {
218            map_bh->b_size = (last_block-block_in_file) << blkbits;
219            if (get_block(inode, block_in_file, map_bh, 0))
220                goto confused;
221            *first_logical_block = block_in_file;
222        }
223
224        if (!buffer_mapped(map_bh)) {
225            fully_mapped = 0;
226            if (first_hole == blocks_per_page)
227                first_hole = page_block;
228            page_block++;
229            block_in_file++;
230            continue;
231        }
232
233        /* some filesystems will copy data into the page during
234         * the get_block call, in which case we don't want to
235         * read it again. map_buffer_to_page copies the data
236         * we just collected from get_block into the page's buffers
237         * so readpage doesn't have to repeat the get_block call
238         */
239        if (buffer_uptodate(map_bh)) {
240            map_buffer_to_page(page, map_bh, page_block);
241            goto confused;
242        }
243    
244        if (first_hole != blocks_per_page)
245            goto confused; /* hole -> non-hole */
246
247        /* Contiguous blocks? */
248        if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
249            goto confused;
250        nblocks = map_bh->b_size >> blkbits;
251        for (relative_block = 0; ; relative_block++) {
252            if (relative_block == nblocks) {
253                clear_buffer_mapped(map_bh);
254                break;
255            } else if (page_block == blocks_per_page)
256                break;
257            blocks[page_block] = map_bh->b_blocknr+relative_block;
258            page_block++;
259            block_in_file++;
260        }
261        bdev = map_bh->b_bdev;
262    }
263
264    if (first_hole != blocks_per_page) {
265        zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
266        if (first_hole == 0) {
267            SetPageUptodate(page);
268            unlock_page(page);
269            goto out;
270        }
271    } else if (fully_mapped) {
272        SetPageMappedToDisk(page);
273    }
274
275    if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
276        cleancache_get_page(page) == 0) {
277        SetPageUptodate(page);
278        goto confused;
279    }
280
281    /*
282     * This page will go to BIO. Do we need to send this BIO off first?
283     */
284    if (bio && (*last_block_in_bio != blocks[0] - 1))
285        bio = mpage_bio_submit(READ, bio);
286
287alloc_new:
288    if (bio == NULL) {
289        bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
290                  min_t(int, nr_pages, bio_get_nr_vecs(bdev)),
291                GFP_KERNEL);
292        if (bio == NULL)
293            goto confused;
294    }
295
296    length = first_hole << blkbits;
297    if (bio_add_page(bio, page, length, 0) < length) {
298        bio = mpage_bio_submit(READ, bio);
299        goto alloc_new;
300    }
301
302    relative_block = block_in_file - *first_logical_block;
303    nblocks = map_bh->b_size >> blkbits;
304    if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
305        (first_hole != blocks_per_page))
306        bio = mpage_bio_submit(READ, bio);
307    else
308        *last_block_in_bio = blocks[blocks_per_page - 1];
309out:
310    return bio;
311
312confused:
313    if (bio)
314        bio = mpage_bio_submit(READ, bio);
315    if (!PageUptodate(page))
316            block_read_full_page(page, get_block);
317    else
318        unlock_page(page);
319    goto out;
320}
321
322/**
323 * mpage_readpages - populate an address space with some pages & start reads against them
324 * @mapping: the address_space
325 * @pages: The address of a list_head which contains the target pages. These
326 * pages have their ->index populated and are otherwise uninitialised.
327 * The page at @pages->prev has the lowest file offset, and reads should be
328 * issued in @pages->prev to @pages->next order.
329 * @nr_pages: The number of pages at *@pages
330 * @get_block: The filesystem's block mapper function.
331 *
332 * This function walks the pages and the blocks within each page, building and
333 * emitting large BIOs.
334 *
335 * If anything unusual happens, such as:
336 *
337 * - encountering a page which has buffers
338 * - encountering a page which has a non-hole after a hole
339 * - encountering a page with non-contiguous blocks
340 *
341 * then this code just gives up and calls the buffer_head-based read function.
342 * It does handle a page which has holes at the end - that is a common case:
343 * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
344 *
345 * BH_Boundary explanation:
346 *
347 * There is a problem. The mpage read code assembles several pages, gets all
348 * their disk mappings, and then submits them all. That's fine, but obtaining
349 * the disk mappings may require I/O. Reads of indirect blocks, for example.
350 *
351 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
352 * submitted in the following order:
353 * 12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
354 *
355 * because the indirect block has to be read to get the mappings of blocks
356 * 13,14,15,16. Obviously, this impacts performance.
357 *
358 * So what we do it to allow the filesystem's get_block() function to set
359 * BH_Boundary when it maps block 11. BH_Boundary says: mapping of the block
360 * after this one will require I/O against a block which is probably close to
361 * this one. So you should push what I/O you have currently accumulated.
362 *
363 * This all causes the disk requests to be issued in the correct order.
364 */
365int
366mpage_readpages(struct address_space *mapping, struct list_head *pages,
367                unsigned nr_pages, get_block_t get_block)
368{
369    struct bio *bio = NULL;
370    unsigned page_idx;
371    sector_t last_block_in_bio = 0;
372    struct buffer_head map_bh;
373    unsigned long first_logical_block = 0;
374    struct blk_plug plug;
375
376    blk_start_plug(&plug);
377
378    map_bh.b_state = 0;
379    map_bh.b_size = 0;
380    for (page_idx = 0; page_idx < nr_pages; page_idx++) {
381        struct page *page = list_entry(pages->prev, struct page, lru);
382
383        prefetchw(&page->flags);
384        list_del(&page->lru);
385        if (!add_to_page_cache_lru(page, mapping,
386                    page->index, GFP_KERNEL)) {
387            bio = do_mpage_readpage(bio, page,
388                    nr_pages - page_idx,
389                    &last_block_in_bio, &map_bh,
390                    &first_logical_block,
391                    get_block);
392        }
393        page_cache_release(page);
394    }
395    BUG_ON(!list_empty(pages));
396    if (bio)
397        mpage_bio_submit(READ, bio);
398    blk_finish_plug(&plug);
399    return 0;
400}
401EXPORT_SYMBOL(mpage_readpages);
402
403/*
404 * This isn't called much at all
405 */
406int mpage_readpage(struct page *page, get_block_t get_block)
407{
408    struct bio *bio = NULL;
409    sector_t last_block_in_bio = 0;
410    struct buffer_head map_bh;
411    unsigned long first_logical_block = 0;
412
413    map_bh.b_state = 0;
414    map_bh.b_size = 0;
415    bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
416            &map_bh, &first_logical_block, get_block);
417    if (bio)
418        mpage_bio_submit(READ, bio);
419    return 0;
420}
421EXPORT_SYMBOL(mpage_readpage);
422
423/*
424 * Writing is not so simple.
425 *
426 * If the page has buffers then they will be used for obtaining the disk
427 * mapping. We only support pages which are fully mapped-and-dirty, with a
428 * special case for pages which are unmapped at the end: end-of-file.
429 *
430 * If the page has no buffers (preferred) then the page is mapped here.
431 *
432 * If all blocks are found to be contiguous then the page can go into the
433 * BIO. Otherwise fall back to the mapping's writepage().
434 *
435 * FIXME: This code wants an estimate of how many pages are still to be
436 * written, so it can intelligently allocate a suitably-sized BIO. For now,
437 * just allocate full-size (16-page) BIOs.
438 */
439
440struct mpage_data {
441    struct bio *bio;
442    sector_t last_block_in_bio;
443    get_block_t *get_block;
444    unsigned use_writepage;
445};
446
447static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
448              void *data)
449{
450    struct mpage_data *mpd = data;
451    struct bio *bio = mpd->bio;
452    struct address_space *mapping = page->mapping;
453    struct inode *inode = page->mapping->host;
454    const unsigned blkbits = inode->i_blkbits;
455    unsigned long end_index;
456    const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
457    sector_t last_block;
458    sector_t block_in_file;
459    sector_t blocks[MAX_BUF_PER_PAGE];
460    unsigned page_block;
461    unsigned first_unmapped = blocks_per_page;
462    struct block_device *bdev = NULL;
463    int boundary = 0;
464    sector_t boundary_block = 0;
465    struct block_device *boundary_bdev = NULL;
466    int length;
467    struct buffer_head map_bh;
468    loff_t i_size = i_size_read(inode);
469    int ret = 0;
470
471    if (page_has_buffers(page)) {
472        struct buffer_head *head = page_buffers(page);
473        struct buffer_head *bh = head;
474
475        /* If they're all mapped and dirty, do it */
476        page_block = 0;
477        do {
478            BUG_ON(buffer_locked(bh));
479            if (!buffer_mapped(bh)) {
480                /*
481                 * unmapped dirty buffers are created by
482                 * __set_page_dirty_buffers -> mmapped data
483                 */
484                if (buffer_dirty(bh))
485                    goto confused;
486                if (first_unmapped == blocks_per_page)
487                    first_unmapped = page_block;
488                continue;
489            }
490
491            if (first_unmapped != blocks_per_page)
492                goto confused; /* hole -> non-hole */
493
494            if (!buffer_dirty(bh) || !buffer_uptodate(bh))
495                goto confused;
496            if (page_block) {
497                if (bh->b_blocknr != blocks[page_block-1] + 1)
498                    goto confused;
499            }
500            blocks[page_block++] = bh->b_blocknr;
501            boundary = buffer_boundary(bh);
502            if (boundary) {
503                boundary_block = bh->b_blocknr;
504                boundary_bdev = bh->b_bdev;
505            }
506            bdev = bh->b_bdev;
507        } while ((bh = bh->b_this_page) != head);
508
509        if (first_unmapped)
510            goto page_is_mapped;
511
512        /*
513         * Page has buffers, but they are all unmapped. The page was
514         * created by pagein or read over a hole which was handled by
515         * block_read_full_page(). If this address_space is also
516         * using mpage_readpages then this can rarely happen.
517         */
518        goto confused;
519    }
520
521    /*
522     * The page has no buffers: map it to disk
523     */
524    BUG_ON(!PageUptodate(page));
525    block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
526    last_block = (i_size - 1) >> blkbits;
527    map_bh.b_page = page;
528    for (page_block = 0; page_block < blocks_per_page; ) {
529
530        map_bh.b_state = 0;
531        map_bh.b_size = 1 << blkbits;
532        if (mpd->get_block(inode, block_in_file, &map_bh, 1))
533            goto confused;
534        if (buffer_new(&map_bh))
535            unmap_underlying_metadata(map_bh.b_bdev,
536                        map_bh.b_blocknr);
537        if (buffer_boundary(&map_bh)) {
538            boundary_block = map_bh.b_blocknr;
539            boundary_bdev = map_bh.b_bdev;
540        }
541        if (page_block) {
542            if (map_bh.b_blocknr != blocks[page_block-1] + 1)
543                goto confused;
544        }
545        blocks[page_block++] = map_bh.b_blocknr;
546        boundary = buffer_boundary(&map_bh);
547        bdev = map_bh.b_bdev;
548        if (block_in_file == last_block)
549            break;
550        block_in_file++;
551    }
552    BUG_ON(page_block == 0);
553
554    first_unmapped = page_block;
555
556page_is_mapped:
557    end_index = i_size >> PAGE_CACHE_SHIFT;
558    if (page->index >= end_index) {
559        /*
560         * The page straddles i_size. It must be zeroed out on each
561         * and every writepage invocation because it may be mmapped.
562         * "A file is mapped in multiples of the page size. For a file
563         * that is not a multiple of the page size, the remaining memory
564         * is zeroed when mapped, and writes to that region are not
565         * written out to the file."
566         */
567        unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
568
569        if (page->index > end_index || !offset)
570            goto confused;
571        zero_user_segment(page, offset, PAGE_CACHE_SIZE);
572    }
573
574    /*
575     * This page will go to BIO. Do we need to send this BIO off first?
576     */
577    if (bio && mpd->last_block_in_bio != blocks[0] - 1)
578        bio = mpage_bio_submit(WRITE, bio);
579
580alloc_new:
581    if (bio == NULL) {
582        bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
583                bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
584        if (bio == NULL)
585            goto confused;
586    }
587
588    /*
589     * Must try to add the page before marking the buffer clean or
590     * the confused fail path above (OOM) will be very confused when
591     * it finds all bh marked clean (i.e. it will not write anything)
592     */
593    length = first_unmapped << blkbits;
594    if (bio_add_page(bio, page, length, 0) < length) {
595        bio = mpage_bio_submit(WRITE, bio);
596        goto alloc_new;
597    }
598
599    /*
600     * OK, we have our BIO, so we can now mark the buffers clean. Make
601     * sure to only clean buffers which we know we'll be writing.
602     */
603    if (page_has_buffers(page)) {
604        struct buffer_head *head = page_buffers(page);
605        struct buffer_head *bh = head;
606        unsigned buffer_counter = 0;
607
608        do {
609            if (buffer_counter++ == first_unmapped)
610                break;
611            clear_buffer_dirty(bh);
612            bh = bh->b_this_page;
613        } while (bh != head);
614
615        /*
616         * we cannot drop the bh if the page is not uptodate
617         * or a concurrent readpage would fail to serialize with the bh
618         * and it would read from disk before we reach the platter.
619         */
620        if (buffer_heads_over_limit && PageUptodate(page))
621            try_to_free_buffers(page);
622    }
623
624    BUG_ON(PageWriteback(page));
625    set_page_writeback(page);
626    unlock_page(page);
627    if (boundary || (first_unmapped != blocks_per_page)) {
628        bio = mpage_bio_submit(WRITE, bio);
629        if (boundary_block) {
630            write_boundary_block(boundary_bdev,
631                    boundary_block, 1 << blkbits);
632        }
633    } else {
634        mpd->last_block_in_bio = blocks[blocks_per_page - 1];
635    }
636    goto out;
637
638confused:
639    if (bio)
640        bio = mpage_bio_submit(WRITE, bio);
641
642    if (mpd->use_writepage) {
643        ret = mapping->a_ops->writepage(page, wbc);
644    } else {
645        ret = -EAGAIN;
646        goto out;
647    }
648    /*
649     * The caller has a ref on the inode, so *mapping is stable
650     */
651    mapping_set_error(mapping, ret);
652out:
653    mpd->bio = bio;
654    return ret;
655}
656
657/**
658 * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
659 * @mapping: address space structure to write
660 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
661 * @get_block: the filesystem's block mapper function.
662 * If this is NULL then use a_ops->writepage. Otherwise, go
663 * direct-to-BIO.
664 *
665 * This is a library function, which implements the writepages()
666 * address_space_operation.
667 *
668 * If a page is already under I/O, generic_writepages() skips it, even
669 * if it's dirty. This is desirable behaviour for memory-cleaning writeback,
670 * but it is INCORRECT for data-integrity system calls such as fsync(). fsync()
671 * and msync() need to guarantee that all the data which was dirty at the time
672 * the call was made get new I/O started against them. If wbc->sync_mode is
673 * WB_SYNC_ALL then we were called for data integrity and we must wait for
674 * existing IO to complete.
675 */
676int
677mpage_writepages(struct address_space *mapping,
678        struct writeback_control *wbc, get_block_t get_block)
679{
680    struct blk_plug plug;
681    int ret;
682
683    blk_start_plug(&plug);
684
685    if (!get_block)
686        ret = generic_writepages(mapping, wbc);
687    else {
688        struct mpage_data mpd = {
689            .bio = NULL,
690            .last_block_in_bio = 0,
691            .get_block = get_block,
692            .use_writepage = 1,
693        };
694
695        ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
696        if (mpd.bio)
697            mpage_bio_submit(WRITE, mpd.bio);
698    }
699    blk_finish_plug(&plug);
700    return ret;
701}
702EXPORT_SYMBOL(mpage_writepages);
703
704int mpage_writepage(struct page *page, get_block_t get_block,
705    struct writeback_control *wbc)
706{
707    struct mpage_data mpd = {
708        .bio = NULL,
709        .last_block_in_bio = 0,
710        .get_block = get_block,
711        .use_writepage = 0,
712    };
713    int ret = __mpage_writepage(page, wbc, &mpd);
714    if (mpd.bio)
715        mpage_bio_submit(WRITE, mpd.bio);
716    return ret;
717}
718EXPORT_SYMBOL(mpage_writepage);
719

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