Re: [PATCH v2] ext4: use fsdata to track inline data write state and fix race

[Aditya Prakash Srivastava]

Hi Jan,

I have been thinking about the VFS retry case where if we do
not reset the fsdata, we might end up in an infinite loop.

See below

________________________________

Phase 1: Preparing the inline write (Thread 1)

VFS entry: Userspace calls write(). The kernel enters
generic_perform_write().

Setup: VFS defines void *fsdata = NULL on the stack.

Begin: ext4_write_begin() is invoked.

The file is inline, so it prepares an inline write.

It sets *fsdata = (void *)EXT4_WRITE_DATA_INLINE.

It returns success (0).

Data copy: VFS copies the userspace data into the page/folio.

________________________________

Phase 2: The race and inline conversion (Thread 2)

Concurrent conversion: While Thread 1 is preparing the write,
Thread 2 triggers a page fault (ext4_page_mkwrite()).

Conversion complete: Thread 2 converts the inode from inline
data to standard extents, clearing
EXT4_STATE_MAY_INLINE_DATA and the inode's inline-data
flag.

________________________________

Phase 3: write_end() and the first retry (Thread 1)

End attempt: VFS calls ext4_write_end().

Inline path: ext4_write_end() sees
fsdata == EXT4_WRITE_DATA_INLINE and routes the write to
ext4_write_inline_data_end().

Conversion detected: ext4_write_inline_data_end() acquires
the xattr lock and checks ext4_has_inline_data(inode).

Since Thread 2 converted the file, the check returns
false.

Retry requested: ext4_write_inline_data_end() releases its
locks/resources and returns 0, telling the VFS that the
write should be retried from the beginning.

________________________________

Phase 4: The VFS retry and the bug (Thread 1)

Retry: generic_perform_write() receives 0 and executes
goto retry;.

Stale fsdata: The retry: label is after the
fsdata = NULL initialization. Therefore:

fsdata is not reset to NULL.

It still contains EXT4_WRITE_DATA_INLINE.

Second begin: ext4_write_begin() is called again.

Since *fsdata = NULL was removed from
ext4_write_begin(), *fsdata is left unchanged and
still contains EXT4_WRITE_DATA_INLINE.

Since the file is no longer inline,
ext4_write_begin() prepares a normal extent-based
write.

Second end: VFS calls ext4_write_end().

It checks whether
fsdata == EXT4_WRITE_DATA_INLINE.

Although this write was prepared as an extent write,
the check is still true because fsdata is stale.

Loop closes: ext4_write_end() incorrectly routes the write
to ext4_write_inline_data_end() again.

It acquires the xattr lock, sees that the inode no
longer contains inline data, releases its resources,
and returns 0 again.

Infinite loop: VFS jumps back to retry:. Since fsdata is
never cleared, the same sequence repeats indefinitely.
________________________________

The proposed fix from my side is to reintroduce setting
fsdata = NULL in the beginning of ext4_write_begin,
of course we should be careful not to overwrite it if it
has some other value. Sample code like below

if (unlikely(ret))
return ret;

/*
 * Reset fsdata for a vfs retry case
*/
if (*fsdata != (void *)FALL_BACK_TO_NONDELALLOC)
*fsdata = NULL;

trace_ext4_write_begin(inode, pos, len);
/*
 * Reserve one block more for addition to orphan list in case

This should take care of the retry case not getting into
an infinite loop.
What do you think? I shall send you a revised version if we
agree on this.

Thanks,
Aditya

On Thu, Jul 2, 2026 at 4:05 PM Jan Kara <jack@xxxxxxx> wrote:
>
> On Thu 02-07-26 05:07:24, Aditya Srivastava wrote:
> > From: Aditya Prakash Srivastava <aditya.ansh182@xxxxxxxxx>
> >
> > Instead of checking the live inode state (ext4_has_inline_data(inode)
> > and ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)) in the
> > write_end handlers, use the fsdata parameter of the address space
> > operations to explicitly pass down the state in which write_begin
> > prepared the write.
> >
> > A concurrent thread (such as ext4_page_mkwrite()) can convert the
> > inline data to an extent between write_begin and write_end. If this
> > happens, the write_end handlers would previously miss the inline
> > write_end path and fall through to extent-based write_end logic.
> > However, since block buffers were never allocated in write_begin,
> > this resulted in NULL pointer dereferences or data loss because
> > folio_buffers(folio) was NULL.
> >
> > Define EXT4_WRITE_DATA_INLINE (3) and communicate this state via
> > fsdata:
> > 1) ext4_write_begin() and ext4_da_write_begin() explicitly set
> >    *fsdata to EXT4_WRITE_DATA_INLINE when an inline write is
> >    successfully prepared.
> > 2) ext4_write_end(), ext4_journalled_write_end(), and
> >    ext4_da_write_end() rely solely on fsdata / write_mode to
> >    invoke ext4_write_inline_data_end().
> >
> > Furthermore, during a buffered write, ext4_write_inline_data_end()
> > acquires the xattr lock after preparing the write. If a concurrent
> > page fault (ext4_page_mkwrite()) converts the inline data to an extent
> > after the write_end handlers check the state but before
> > ext4_write_inline_data_end() acquires the xattr write lock, the
> > subsequent check will trigger a kernel panic via
> > BUG_ON(!ext4_has_inline_data(inode)).
> >
> > Replace the BUG_ON check in ext4_write_inline_data_end() with a graceful
> > error-handling retry path. If the inline data is cleared after locking
> > the xattr, we safely release all resources (releasing iloc.bh,
> > unlocking/putting the folio, stopping the active journal transaction
> > handle) and return 0 (VFS retry) to let the generic write path retry
> > the operation safely.
> >
> > Reported-by: syzbot+0c89d865531d053abb2d@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=0c89d865531d053abb2d
> > Fixes: 3fdcfb668fd7 ("ext4: add journalled write support for inline data")
> > Suggested-by: Jan Kara <jack@xxxxxxx>
> > Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@xxxxxxxxx>
>
> Looks good! Feel free to add:
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
>
>                                                                 Honza
>
> > ---
> > Changes in v2:
> > - Folded the BUG_ON fix from the second patch into the first one to
> >   ensure bisectability across git history, as suggested by Jan Kara.
> > - Removed the pointless initialization `*fsdata = NULL` on entry to
> >   `ext4_write_begin()`.
> > - Removed the redundant check `if (fsdata)` in `ext4_write_begin()`.
> >
> >  fs/ext4/ext4.h   |  1 +
> >  fs/ext4/inline.c | 14 +++++++++++++-
> >  fs/ext4/inode.c  | 18 +++++++++---------
> >  3 files changed, 23 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index b37c136ea3ab..521bd5d6321c 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -3138,6 +3138,7 @@ int do_journal_get_write_access(handle_t *handle, struct inode *inode,
> >  void ext4_set_inode_mapping_order(struct inode *inode);
> >  #define FALL_BACK_TO_NONDELALLOC 1
> >  #define CONVERT_INLINE_DATA   2
> > +#define EXT4_WRITE_DATA_INLINE        3
> >
> >  typedef enum {
> >       EXT4_IGET_NORMAL =      0,
> > diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
> > index 8045e4ff270c..cfd591dc1d9c 100644
> > --- a/fs/ext4/inline.c
> > +++ b/fs/ext4/inline.c
> > @@ -812,7 +812,19 @@ int ext4_write_inline_data_end(struct inode *inode, loff_t pos, unsigned len,
> >                       goto out;
> >               }
> >               ext4_write_lock_xattr(inode, &no_expand);
> > -             BUG_ON(!ext4_has_inline_data(inode));
> > +             /*
> > +              * We could have raced with ext4_page_mkwrite() converting
> > +              * the inode and clearing the inline data flag, so we just
> > +              * release resources and retry the whole write.
> > +              */
> > +             if (unlikely(!ext4_has_inline_data(inode))) {
> > +                     ext4_write_unlock_xattr(inode, &no_expand);
> > +                     brelse(iloc.bh);
> > +                     folio_unlock(folio);
> > +                     folio_put(folio);
> > +                     ext4_journal_stop(handle);
> > +                     return 0;
> > +             }
> >
> >               /*
> >                * ei->i_inline_off may have changed since
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index ce99807c5f5b..d3d9e1999670 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -1316,8 +1316,10 @@ static int ext4_write_begin(const struct kiocb *iocb,
> >                                                   foliop);
> >               if (ret < 0)
> >                       return ret;
> > -             if (ret == 1)
> > +             if (ret == 1) {
> > +                     *fsdata = (void *)EXT4_WRITE_DATA_INLINE;
> >                       return 0;
> > +             }
> >       }
> >
> >       /*
> > @@ -1450,8 +1452,7 @@ static int ext4_write_end(const struct kiocb *iocb,
> >
> >       trace_ext4_write_end(inode, pos, len, copied);
> >
> > -     if (ext4_has_inline_data(inode) &&
> > -         ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA))
> > +     if (fsdata == (void *)EXT4_WRITE_DATA_INLINE)
> >               return ext4_write_inline_data_end(inode, pos, len, copied,
> >                                                 folio);
> >
> > @@ -1560,8 +1561,7 @@ static int ext4_journalled_write_end(const struct kiocb *iocb,
> >
> >       BUG_ON(!ext4_handle_valid(handle));
> >
> > -     if (ext4_has_inline_data(inode) &&
> > -         ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA))
> > +     if (fsdata == (void *)EXT4_WRITE_DATA_INLINE)
> >               return ext4_write_inline_data_end(inode, pos, len, copied,
> >                                                 folio);
> >
> > @@ -3161,8 +3161,10 @@ static int ext4_da_write_begin(const struct kiocb *iocb,
> >                                                    foliop, fsdata, true);
> >               if (ret < 0)
> >                       return ret;
> > -             if (ret == 1)
> > +             if (ret == 1) {
> > +                     *fsdata = (void *)EXT4_WRITE_DATA_INLINE;
> >                       return 0;
> > +             }
> >       }
> >
> >  retry:
> > @@ -3299,9 +3301,7 @@ static int ext4_da_write_end(const struct kiocb *iocb,
> >
> >       trace_ext4_da_write_end(inode, pos, len, copied);
> >
> > -     if (write_mode != CONVERT_INLINE_DATA &&
> > -         ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA) &&
> > -         ext4_has_inline_data(inode))
> > +     if (write_mode == EXT4_WRITE_DATA_INLINE)
> >               return ext4_write_inline_data_end(inode, pos, len, copied,
> >                                                 folio);
> >
> > --
> > 2.47.3
> >
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR