On Tue, Oct 29, 2019 at 02:48:50PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> AIO+DIO can extend the file size on IO completion, and it holds
> no inode locks while the IO is in flight. Therefore, a race
> condition exists in file size updates if we do something like this:
>
> aio-thread fallocate-thread
>
> lock inode
> submit IO beyond inode->i_size
> unlock inode
> .....
> lock inode
> break layouts
> if (off + len > inode->i_size)
> new_size = off + len
> .....
> inode_dio_wait()
> <blocks>
> .....
> completes
> inode->i_size updated
> inode_dio_done()
> ....
> <wakes>
> <does stuff no long beyond EOF>
> if (new_size)
> xfs_vn_setattr(inode, new_size)
>
>
> Yup, that attempt to extend the file size in the fallocate code
> turns into a truncate - it removes the whatever the aio write
> allocated and put to disk, and reduced the inode size back down to
> where the fallocate operation ends.
>
> Fundamentally, xfs_file_fallocate() not compatible with racing
> AIO+DIO completions, so we need to move the inode_dio_wait() call
> up to where the lock the inode and break the layouts.
>
> Secondly, storing the inode size and then using it unchecked without
> holding the ILOCK is not safe; we can only do such a thing if we've
> locked out and drained all IO and other modification operations,
> which we don't do initially in xfs_file_fallocate.
>
> It should be noted that some of the fallocate operations are
> compound operations - they are made up of multiple manipulations
> that may zero data, and so we may need to flush and invalidate the
> file multiple times during an operation. However, we only need to
> lock out IO and other space manipulation operations once, as that
> lockout is maintained until the entire fallocate operation has been
> completed.
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/xfs_bmap_util.c | 8 +-------
> fs/xfs/xfs_file.c | 30 ++++++++++++++++++++++++++++++
> fs/xfs/xfs_ioctl.c | 1 +
> 3 files changed, 32 insertions(+), 7 deletions(-)
>
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index fb31d7d6701e..dea68308fb64 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -1040,6 +1040,7 @@ xfs_unmap_extent(
> goto out_unlock;
> }
>
> +/* Caller must first wait for the completion of any pending DIOs if required. */
> int
> xfs_flush_unmap_range(
> struct xfs_inode *ip,
> @@ -1051,9 +1052,6 @@ xfs_flush_unmap_range(
> xfs_off_t rounding, start, end;
> int error;
>
> - /* wait for the completion of any pending DIOs */
> - inode_dio_wait(inode);
Does xfs_reflink_remap_prep still need this function to call inode_dio_wait
before zapping the page cache prior to reflinking into an existing file?
> -
> rounding = max_t(xfs_off_t, 1 << mp->m_sb.sb_blocklog, PAGE_SIZE);
> start = round_down(offset, rounding);
> end = round_up(offset + len, rounding) - 1;
> @@ -1085,10 +1083,6 @@ xfs_free_file_space(
> if (len <= 0) /* if nothing being freed */
> return 0;
>
> - error = xfs_flush_unmap_range(ip, offset, len);
> - if (error)
> - return error;
> -
> startoffset_fsb = XFS_B_TO_FSB(mp, offset);
> endoffset_fsb = XFS_B_TO_FSBT(mp, offset + len);
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 525b29b99116..865543e41fb4 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -817,6 +817,36 @@ xfs_file_fallocate(
> if (error)
> goto out_unlock;
>
> + /*
> + * Must wait for all AIO to complete before we continue as AIO can
> + * change the file size on completion without holding any locks we
> + * currently hold. We must do this first because AIO can update both
> + * the on disk and in memory inode sizes, and the operations that follow
> + * require the in-memory size to be fully up-to-date.
> + */
> + inode_dio_wait(inode);
> +
> + /*
> + * Now AIO and DIO has drained we flush and (if necessary) invalidate
> + * the cached range over the first operation we are about to run.
> + *
> + * We care about zero and collapse here because they both run a hole
> + * punch over the range first. Because that can zero data, and the range
> + * of invalidation for the shift operations is much larger, we still do
> + * the required flush for collapse in xfs_prepare_shift().
> + *
> + * Insert has the same range requirements as collapse, and we extend the
> + * file first which can zero data. Hence insert has the same
> + * flush/invalidate requirements as collapse and so they are both
> + * handled at the right time by xfs_prepare_shift().
> + */
> + if (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE |
> + FALLOC_FL_COLLAPSE_RANGE)) {
Er... "Insert has the same requirements as collapse", but we don't test
for that here? Also ... xfs_prepare_shift handles flushing for both
collapse and insert range, but we still have to flush here for collapse?
<confused but suspecting this has something to do with the fact that we
only do insert range after updating the isize?>
I think the third paragraph of the comment is just confusing me more.
Does the following describe what's going on?
"Insert range has the same range [should this be "page cache flushing"?]
requirements as collapse. Because we can zero data as part of extending
the file size, we skip the flush here and let the flush in
xfs_prepare_shift take care of invalidating the page cache." ?
--D
> + error = xfs_flush_unmap_range(ip, offset, len);
> + if (error)
> + goto out_unlock;
> + }
> +
> if (mode & FALLOC_FL_PUNCH_HOLE) {
> error = xfs_free_file_space(ip, offset, len);
> if (error)
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 287f83eb791f..800f07044636 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -623,6 +623,7 @@ xfs_ioc_space(
> error = xfs_break_layouts(inode, &iolock, BREAK_UNMAP);
> if (error)
> goto out_unlock;
> + inode_dio_wait(inode);
>
> switch (bf->l_whence) {
> case 0: /*SEEK_SET*/
> --
> 2.24.0.rc0
>
