Re: [PATCH 3/5] xfs: restrict when we try to align cow fork delalloc to cowextsz hints

Darrick J. Wong
2024-06-14T05:27:05Z

On Fri, Jun 14, 2024 at 06:41:55AM +0200, Christoph Hellwig wrote:
> On Thu, Jun 13, 2024 at 09:13:10PM -0700, Darrick J. Wong wrote:
> > > Looking at the caller below I don't think we need the use_cowextszhint
> > > flag here, we can just locally check for prealloc beeing non-0 in
> > > the branch below:
> >
> > That won't work, because xfs_buffered_write_iomap_begin only sets
> > @prealloc to nonzero if it thinks is an extending write. For the cow
> > fork, we create delalloc reservations that are aligned to the cowextsize
> > value for overwrites below eof.
>
> Yeah. For that we'd need to move the retry loop into
> xfs_bmapi_reserve_delalloc - which honestly feels like the more logical
> place for it anyway. As in the untested version below, also note
> my XXX comment about a comment being added:
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index c101cf266bc4db..58ff21cb84e0f5 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -4059,19 +4059,33 @@ xfs_bmapi_reserve_delalloc(
> uint64_t fdblocks;
> int error;
> xfs_fileoff_t aoff = off;
> + bool use_cowextszhint =
> + whichfork == XFS_COW_FORK && !prealloc;
>
> /*
> * Cap the alloc length. Keep track of prealloc so we know whether to
> * tag the inode before we return.
> */
> +retry:
> alen = XFS_FILBLKS_MIN(len + prealloc, XFS_MAX_BMBT_EXTLEN);
> if (!eof)
> alen = XFS_FILBLKS_MIN(alen, got->br_startoff - aoff);
> if (prealloc && alen >= len)
> prealloc = alen - len;
>
> - /* Figure out the extent size, adjust alen */
> - if (whichfork == XFS_COW_FORK) {
> + /*
> + * If we're targeting the COW fork but aren't creating a speculative
> + * posteof preallocation, try to expand the reservation to align with
> + * the cow extent size hint if there's sufficient free space.
> + *
> + * Unlike the data fork, the CoW cancellation functions will free all
> + * the reservations at inactivation, so we don't require that every
> + * delalloc reservation have a dirty pagecache.
> + *
> + * XXX(hch): I can't see where we actually require dirty pagecache
> + * for speculative data fork preallocations. What am I missing?

IIRC a delalloc reservation in the data fork that isn't backing a dirty
page will just sit there in the data fork and never get reclaimed.
There's no writeback to turn it into an unwritten -> written extent.
The blockgc functions won't (can't?) walk the pagecache to find clean
regions that could be torn down. xfs destroy_inode just asserts on any
reservations that it finds.

--D

> + */
> + if (use_cowextszhint) {
> struct xfs_bmbt_irec prev;
> xfs_extlen_t extsz = xfs_get_cowextsz_hint(ip);
>
> @@ -4090,7 +4104,7 @@ xfs_bmapi_reserve_delalloc(
> */
> error = xfs_quota_reserve_blkres(ip, alen);
> if (error)
> - return error;
> + goto out;
>
> /*
> * Split changing sb for alen and indlen since they could be coming
> @@ -4140,6 +4154,16 @@ xfs_bmapi_reserve_delalloc(
> out_unreserve_quota:
> if (XFS_IS_QUOTA_ON(mp))
> xfs_quota_unreserve_blkres(ip, alen);
> +out:
> + if (error == -ENOSPC || error == -EDQUOT) {
> + trace_xfs_delalloc_enospc(ip, off, len);
> + if (prealloc || use_cowextszhint) {
> + /* retry without any preallocation */
> + prealloc = 0;
> + use_cowextszhint = false;
> + goto retry;
> + }
> + }
> return error;
> }
>
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 3783426739258c..34cce017fe7ce1 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1148,27 +1148,13 @@ xfs_buffered_write_iomap_begin(
> }
> }
>
> -retry:
> error = xfs_bmapi_reserve_delalloc(ip, allocfork, offset_fsb,
> end_fsb - offset_fsb, prealloc_blocks,
> allocfork == XFS_DATA_FORK ? &imap : &cmap,
> allocfork == XFS_DATA_FORK ? &icur : &ccur,
> allocfork == XFS_DATA_FORK ? eof : cow_eof);
> - switch (error) {
> - case 0:
> - break;
> - case -ENOSPC:
> - case -EDQUOT:
> - /* retry without any preallocation */
> - trace_xfs_delalloc_enospc(ip, offset, count);
> - if (prealloc_blocks) {
> - prealloc_blocks = 0;
> - goto retry;
> - }
> - fallthrough;
> - default:
> + if (error)
> goto out_unlock;
> - }
>
> if (allocfork == XFS_COW_FORK) {
> trace_xfs_iomap_alloc(ip, offset, count, allocfork, &cmap);