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

From: Jiayuan Chen

Date: Thu Feb 26 2026 - 09:43:51 EST

February 26, 2026 at 21:21, "Jan Kara" <jack@xxxxxxx mailto:jack@xxxxxxx?to=%22Jan%20Kara%22%20%3Cjack%40suse.cz%3E > wrote:

>
> On Thu 26-02-26 16:40:07, Jiayuan Chen wrote:
>
> >
> > From: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Link: https://syzkaller.appspot.com/bug?extid=6880f676b265dbd42d63
> > Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
> >
> Given this, I think it would be much less intrusive and also more
> explanatory to just mark prev_pos with __data_racy with appropriate reason
> you're mentioning in the changelog.

Thanks for the suggestion. I'm fine either way — __data_racy is indeed
cleaner and less intrusive for a purely heuristic field like this.

I'll wait a bit to see if Andrew or other mm folks have a preference
before resending. Happy to go with whichever approach they prefer.

> Honza
>
> >
> > ---
> > 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
> >
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR
>