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BUG: KCSAN: data-race in ext4_writepages / xas_clear_mark

read to 0xffff8880b6e35854 of 4 bytes by task 4529 on cpu 0:
 mapping_tagged include/linux/fs.h:521 [inline]
 ext4_writepages+0x1b4/0x2300 fs/ext4/inode.c:2736
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __filemap_fdatawrite_range+0x1c5/0x230 mm/filemap.c:420
 filemap_write_and_wait_range mm/filemap.c:680 [inline]
 filemap_write_and_wait_range+0xb1/0x150 mm/filemap.c:674
 generic_file_direct_write+0x27a/0x2e0 mm/filemap.c:3183
 __generic_file_write_iter+0x17e/0x380 mm/filemap.c:3390
 ext4_file_write_iter+0x1bd/0xa00 fs/ext4/file.c:268
 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
 do_splice_direct+0x161/0x1e0 fs/splice.c:1063

write to 0xffff8880b6e35854 of 4 bytes by task 4530 on cpu 1:
 xa_mark_clear lib/xarray.c:74 [inline]
 xas_clear_mark lib/xarray.c:917 [inline]
 xas_clear_mark+0x1a0/0x1d0 lib/xarray.c:898
 __test_set_page_writeback+0x5d3/0x7d0 mm/page-writeback.c:2800
 set_page_writeback include/linux/page-flags.h:542 [inline]
 ext4_bio_write_page+0x157/0xb90 fs/ext4/page-io.c:424
 mpage_submit_page+0xeb/0x170 fs/ext4/inode.c:2222
 mpage_process_page_bufs+0x3b4/0x3d0 fs/ext4/inode.c:2336
 mpage_prepare_extent_to_map+0x48b/0x760 fs/ext4/inode.c:2694
 ext4_writepages+0x94f/0x2300 fs/ext4/inode.c:2822
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 4530 Comm: syz-executor.3 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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