[PATCH v1] mm: annotate data race of f_ra.prev_pos

Jiayuan Chen
2026-02-26T08:40:50Z

From: Jiayuan Chen <jiayuan.chen@shopee.com>

KCSAN reports a data race when concurrent readers access the same
struct file:

BUG: KCSAN: data-race in filemap_read / filemap_splice_read

write to 0xffff88811a6f8228 of 8 bytes by task 10061 on cpu 0:
filemap_splice_read+0x523/0x780 mm/filemap.c:3125
...

write to 0xffff88811a6f8228 of 8 bytes by task 10066 on cpu 1:
filemap_read+0x98d/0xa10 mm/filemap.c:2873
...

Both filemap_read() and filemap_splice_read() update f_ra.prev_pos
without synchronization. This is a benign race since prev_pos is only
used as a hint for readahead heuristics in page_cache_sync_ra(), and a
stale or torn value merely results in a suboptimal readahead decision,
not a correctness issue.

Use WRITE_ONCE/READ_ONCE to annotate all accesses to prev_pos across
the tree for consistency and silence KCSAN.

Reported-by: syzbot+6880f676b265dbd42d63@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=6880f676b265dbd42d63
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
fs/ext4/dir.c | 2 +-
fs/ntfs3/fsntfs.c | 2 +-
include/trace/events/readahead.h | 2 +-
mm/filemap.c | 6 +++---
mm/readahead.c | 4 ++--
mm/shmem.c | 2 +-
6 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 28b2a3deb954..1ddf7acce5ca 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -200,7 +200,7 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
sb->s_bdev->bd_mapping,
&file->f_ra, file, index,
1 << EXT4_SB(sb)->s_min_folio_order);
- file->f_ra.prev_pos = (loff_t)index << PAGE_SHIFT;
+ WRITE_ONCE(file->f_ra.prev_pos, (loff_t)index << PAGE_SHIFT);
bh = ext4_bread(NULL, inode, map.m_lblk, 0);
if (IS_ERR(bh)) {
err = PTR_ERR(bh);
diff --git a/fs/ntfs3/fsntfs.c b/fs/ntfs3/fsntfs.c
index 0df2aa81d884..d1232fc03c08 100644
--- a/fs/ntfs3/fsntfs.c
+++ b/fs/ntfs3/fsntfs.c
@@ -1239,7 +1239,7 @@ int ntfs_read_run_nb_ra(struct ntfs_sb_info *sbi, const struct runs_tree *run,
if (!ra_has_index(ra, index)) {
page_cache_sync_readahead(mapping, ra, NULL,
index, 1);
- ra->prev_pos = (loff_t)index << PAGE_SHIFT;
+ WRITE_ONCE(ra->prev_pos, (loff_t)index << PAGE_SHIFT);
}
}

diff --git a/include/trace/events/readahead.h b/include/trace/events/readahead.h
index 0997ac5eceab..63d8df6c2983 100644
--- a/include/trace/events/readahead.h
+++ b/include/trace/events/readahead.h
@@ -101,7 +101,7 @@ DECLARE_EVENT_CLASS(page_cache_ra_op,
__entry->async_size = ra->async_size;
__entry->ra_pages = ra->ra_pages;
__entry->mmap_miss = ra->mmap_miss;
- __entry->prev_pos = ra->prev_pos;
+ __entry->prev_pos = READ_ONCE(ra->prev_pos);
__entry->req_count = req_count;
),

diff --git a/mm/filemap.c b/mm/filemap.c
index 63f256307fdd..d3e2d4b826b9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2771,7 +2771,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
int i, error = 0;
bool writably_mapped;
loff_t isize, end_offset;
- loff_t last_pos = ra->prev_pos;
+ loff_t last_pos = READ_ONCE(ra->prev_pos);

if (unlikely(iocb->ki_pos < 0))
return -EINVAL;
@@ -2870,7 +2870,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
} while (iov_iter_count(iter) && iocb->ki_pos < isize && !error);

file_accessed(filp);
- ra->prev_pos = last_pos;
+ WRITE_ONCE(ra->prev_pos, last_pos);
return already_read ? already_read : error;
}
EXPORT_SYMBOL_GPL(filemap_read);
@@ -3122,7 +3122,7 @@ ssize_t filemap_splice_read(struct file *in, loff_t *ppos,
len -= n;
total_spliced += n;
*ppos += n;
- in->f_ra.prev_pos = *ppos;
+ WRITE_ONCE(in->f_ra.prev_pos, *ppos);
if (pipe_is_full(pipe))
goto out;
}
diff --git a/mm/readahead.c b/mm/readahead.c
index 7b05082c89ea..de49b35b0329 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -142,7 +142,7 @@ void
file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
{
ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
- ra->prev_pos = -1;
+ WRITE_ONCE(ra->prev_pos, -1);
}
EXPORT_SYMBOL_GPL(file_ra_state_init);

@@ -584,7 +584,7 @@ void page_cache_sync_ra(struct readahead_control *ractl,
}

max_pages = ractl_max_pages(ractl, req_count);
- prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
+ prev_index = (unsigned long long)READ_ONCE(ra->prev_pos) >> PAGE_SHIFT;
/*
* A start of file, oversized read, or sequential cache miss:
* trivial case: (index - prev_index) == 1
diff --git a/mm/shmem.c b/mm/shmem.c
index 5e7dcf5bc5d3..03569199baf4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3642,7 +3642,7 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
len -= n;
total_spliced += n;
*ppos += n;
- in->f_ra.prev_pos = *ppos;
+ WRITE_ONCE(in->f_ra.prev_pos, *ppos);
if (pipe_is_full(pipe))
break;

--
2.43.0