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

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