mapping->nrpages is updated under the i_pages lock but is read locklessly as a cheap "anything cached?" hint, e.g. in invalidate_bdev() and filemap_write_and_wait_range(). KCSAN reports the race: BUG: KCSAN: data-race in __filemap_add_folio / invalidate_bdev read-write to 0xffff8881004c4178 of 8 bytes by task 3600 on cpu 0: __filemap_add_folio+0x6b7/0x910 mm/filemap.c:919 filemap_add_folio+0x1cd/0x380 page_cache_ra_unbounded+0x1a4/0x3a0 ... blkdev_read_iter+0x21e/0x2d0 read to 0xffff8881004c4178 of 8 bytes by task 3601 on cpu 1: invalidate_bdev+0x25/0x70 block/bdev.c:98 ext4_fill_super+0x2ccb/0x37b0 The same counter also races with filemap_write_and_wait_range() through mapping_needs_writeback(). The race is benign: a stale value only makes the caller do or skip a harmless invalidate/flush pass. Mark the lockless reads with data_race() in invalidate_bdev() and in the two callers of mapping_needs_writeback(), rather than inside the helper, so a future user needing a reliable answer is not silently covered. Reported-by: syzbot+e0624b5bf72b99131402@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=e0624b5bf72b99131402 Signed-off-by: Nguyen Ngoc Thang --- v3: move data_race() from mapping_needs_writeback() to its two callers (Jan) v2: use data_race() on the readers instead of READ_ONCE()/WRITE_ONCE() (Jan) block/bdev.c | 3 ++- mm/filemap.c | 6 ++++-- 2 files changed, 6 insertions(+), 3 deletions(-) diff --git a/block/bdev.c b/block/bdev.c index cd8323083740..233750f87e6d 100644 --- a/block/bdev.c +++ b/block/bdev.c @@ -95,7 +95,8 @@ void invalidate_bdev(struct block_device *bdev) { struct address_space *mapping = bdev->bd_mapping; - if (mapping->nrpages) { + /* Lockless hint; a stale value only costs or skips a harmless pass */ + if (data_race(mapping->nrpages)) { invalidate_bh_lrus(); lru_add_drain_all(); /* make sure all lru add caches are flushed */ invalidate_mapping_pages(mapping, 0, -1); diff --git a/mm/filemap.c b/mm/filemap.c index 00fd89cf6f55..ff10ac8f56b3 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -680,7 +680,8 @@ int filemap_write_and_wait_range(struct address_space *mapping, if (lend < lstart) return 0; - if (mapping_needs_writeback(mapping)) { + /* Lockless hint; a stale value is harmless */ + if (data_race(mapping_needs_writeback(mapping))) { err = filemap_fdatawrite_range(mapping, lstart, lend); /* * Even if the above returned error, the pages may be @@ -782,7 +783,8 @@ int file_write_and_wait_range(struct file *file, loff_t lstart, loff_t lend) if (lend < lstart) return 0; - if (mapping_needs_writeback(mapping)) { + /* Lockless hint; a stale value is harmless */ + if (data_race(mapping_needs_writeback(mapping))) { err = filemap_fdatawrite_range(mapping, lstart, lend); /* See comment of filemap_write_and_wait() */ if (err != -EIO) -- 2.43.0