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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

read to 0xffff888125813d90 of 8 bytes by task 12833 on cpu 0:
 ext4_writepages+0x188/0x2300 fs/ext4/inode.c:2736
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __filemap_fdatawrite_range+0x1c5/0x230 mm/filemap.c:420
 file_write_and_wait_range+0xfd/0x160 mm/filemap.c:781
 __generic_file_fsync+0x59/0x190 fs/libfs.c:1000
 ext4_sync_file+0x48e/0xb00 fs/ext4/fsync.c:120
 vfs_fsync_range+0x82/0x150 fs/sync.c:197
 generic_write_sync include/linux/fs.h:2853 [inline]
 ext4_file_write_iter+0x45b/0xa00 fs/ext4/file.c:279
 call_write_iter include/linux/fs.h:1895 [inline]
 do_iter_readv_writev+0x487/0x5b0 fs/read_write.c:693
 do_iter_write fs/read_write.c:970 [inline]
 do_iter_write+0x13b/0x3c0 fs/read_write.c:951
 vfs_iter_write+0x5c/0x80 fs/read_write.c:983
 iter_file_splice_write+0x4c0/0x7f0 fs/splice.c:746
 do_splice_from fs/splice.c:848 [inline]
 direct_splice_actor+0xa0/0xc0 fs/splice.c:1020
 splice_direct_to_actor+0x215/0x510 fs/splice.c:975

write to 0xffff888125813d90 of 8 bytes by task 12825 on cpu 1:
 __add_to_page_cache_locked+0x298/0x780 mm/filemap.c:889
 add_to_page_cache_lru+0xe2/0x2d0 mm/filemap.c:942
 ext4_mpage_readpages+0x3ab/0x12c0 fs/ext4/readpage.c:247
 ext4_readpages+0x92/0xc0 fs/ext4/inode.c:3342
 read_pages+0xa2/0x2d0 mm/readahead.c:126
 __do_page_cache_readahead+0x353/0x390 mm/readahead.c:212
 ra_submit mm/internal.h:62 [inline]
 ondemand_readahead+0x35d/0x710 mm/readahead.c:492
 page_cache_async_readahead+0x22c/0x250 mm/readahead.c:574
 generic_file_buffered_read mm/filemap.c:2058 [inline]
 generic_file_read_iter+0xffc/0x1440 mm/filemap.c:2323
 ext4_file_read_iter fs/ext4/file.c:77 [inline]
 ext4_file_read_iter+0xfa/0x240 fs/ext4/file.c:65
 call_read_iter include/linux/fs.h:1889 [inline]
 generic_file_splice_read+0x35c/0x500 fs/splice.c:307
 do_splice_to+0xf2/0x130 fs/splice.c:877
 splice_direct_to_actor+0x1a1/0x510 fs/splice.c:954

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12825 Comm: syz-executor.2 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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