On Thu, Apr 13, 2017 at 02:13:25PM +0200, Christoph Hellwig wrote:
> On Wed, Apr 12, 2017 at 08:52:50PM -0700, Darrick J. Wong wrote:
> > Hmm... can you try out the attached RFC(RAP) xfstests testcase to see if
> > it can reproduce the problem you're seeing, and then apply the (equally
> > RFCRAP) patch to see if it fixes the problem?
>
> Yeah. limiting at the caller seems much better than second guessing
> later. Below is a version of your patch with a couple random edits.
> I wonder if we could now get rid of xfs_refcount_still_have_space
> or at least turn it into warnings only..
We also have to plumb in the code necessary to recover a block unmap log
intent item of arbitrary length by splitting the unmap operation into a
series of __xfs_bunmapi calls. That seems to fix the asserts and other
weird log burps... will send an RFC patch shortly.
--D
>
> ---
> From 9b25b1bdba8fb93650710444a6bab7b10f74680d Mon Sep 17 00:00:00 2001
> From: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
> Date: Thu, 13 Apr 2017 13:35:00 +0200
> Subject: xfs: try to avoid blowing out the transaction reservation when
> bunmaping a shared extent
>
> In a pathological scenario where we are trying to bunmapi a single
> extent in which every other block is shared, it's possible that trying
> to unmap the entire large extent in a single transaction can generate so
> many EFIs that we overflow the transaction reservation.
>
> Therefore, use a heuristic to guess at the number of blocks we can
> safely unmap from a reflink file's data fork in an single transaction.
> This should prevent problems such as the log head slamming into the tail
> and ASSERTs that trigger because we've exceeded the transaction
> reservation.
>
> Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> [hch: random edits, all bugs are my fault]
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> fs/xfs/libxfs/xfs_bmap.c | 23 ++++++++++++++++++++++-
> fs/xfs/libxfs/xfs_refcount.c | 10 +---------
> fs/xfs/libxfs/xfs_refcount.h | 16 ++++++++++++++++
> 3 files changed, 39 insertions(+), 10 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 8e94030bcb8f..04dac8954425 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -5442,6 +5442,7 @@ __xfs_bunmapi(
> int whichfork; /* data or attribute fork */
> xfs_fsblock_t sum;
> xfs_filblks_t len = *rlen; /* length to unmap in file */
> + xfs_fileoff_t max_len;
>
> trace_xfs_bunmap(ip, bno, len, flags, _RET_IP_);
>
> @@ -5463,6 +5464,16 @@ __xfs_bunmapi(
> ASSERT(len > 0);
> ASSERT(nexts >= 0);
>
> + /*
> + * Guesstimate how many blocks we can unmap without running the risk of
> + * blowing out the transaction with a mix of EFIs and reflink
> + * adjustments.
> + */
> + if (xfs_is_reflink_inode(ip) && whichfork == XFS_DATA_FORK)
> + max_len = min(len, xfs_refcount_max_unmap(tp->t_log_res));
> + else
> + max_len = len;
> +
> if (!(ifp->if_flags & XFS_IFEXTENTS) &&
> (error = xfs_iread_extents(tp, ip, whichfork)))
> return error;
> @@ -5507,7 +5518,7 @@ __xfs_bunmapi(
>
> extno = 0;
> while (bno != (xfs_fileoff_t)-1 && bno >= start && lastx >= 0 &&
> - (nexts == 0 || extno < nexts)) {
> + (nexts == 0 || extno < nexts) && max_len > 0) {
> /*
> * Is the found extent after a hole in which bno lives?
> * Just back up to the previous extent, if so.
> @@ -5539,6 +5550,15 @@ __xfs_bunmapi(
> }
> if (del.br_startoff + del.br_blockcount > bno + 1)
> del.br_blockcount = bno + 1 - del.br_startoff;
> +
> + /* How much can we safely unmap? */
> + if (max_len < del.br_blockcount) {
> + del.br_startoff += del.br_blockcount - max_len;
> + if (!wasdel)
> + del.br_startblock += del.br_blockcount - max_len;
> + del.br_blockcount = max_len;
> + }
> +
> sum = del.br_startblock + del.br_blockcount;
> if (isrt &&
> (mod = do_mod(sum, mp->m_sb.sb_rextsize))) {
> @@ -5715,6 +5735,7 @@ __xfs_bunmapi(
> if (!isrt && wasdel)
> xfs_mod_fdblocks(mp, (int64_t)del.br_blockcount, false);
>
> + max_len -= del.br_blockcount;
> bno = del.br_startoff - 1;
> nodelete:
> /*
> diff --git a/fs/xfs/libxfs/xfs_refcount.c b/fs/xfs/libxfs/xfs_refcount.c
> index b177ef33cd4c..29dcde381505 100644
> --- a/fs/xfs/libxfs/xfs_refcount.c
> +++ b/fs/xfs/libxfs/xfs_refcount.c
> @@ -784,14 +784,6 @@ xfs_refcount_merge_extents(
> }
>
> /*
> - * While we're adjusting the refcounts records of an extent, we have
> - * to keep an eye on the number of extents we're dirtying -- run too
> - * many in a single transaction and we'll exceed the transaction's
> - * reservation and crash the fs. Each record adds 12 bytes to the
> - * log (plus any key updates) so we'll conservatively assume 24 bytes
> - * per record. We must also leave space for btree splits on both ends
> - * of the range and space for the CUD and a new CUI.
> - *
> * XXX: This is a pretty hand-wavy estimate. The penalty for guessing
> * true incorrectly is a shutdown FS; the penalty for guessing false
> * incorrectly is more transaction rolls than might be necessary.
> @@ -822,7 +814,7 @@ xfs_refcount_still_have_space(
> else if (overhead > cur->bc_tp->t_log_res)
> return false;
> return cur->bc_tp->t_log_res - overhead >
> - cur->bc_private.a.priv.refc.nr_ops * 32;
> + cur->bc_private.a.priv.refc.nr_ops * XFS_REFCOUNT_ITEM_OVERHEAD;
> }
>
> /*
> diff --git a/fs/xfs/libxfs/xfs_refcount.h b/fs/xfs/libxfs/xfs_refcount.h
> index 098dc668ab2c..eafb9d1f3b37 100644
> --- a/fs/xfs/libxfs/xfs_refcount.h
> +++ b/fs/xfs/libxfs/xfs_refcount.h
> @@ -67,4 +67,20 @@ extern int xfs_refcount_free_cow_extent(struct xfs_mount *mp,
> extern int xfs_refcount_recover_cow_leftovers(struct xfs_mount *mp,
> xfs_agnumber_t agno);
>
> +/*
> + * While we're adjusting the refcounts records of an extent, we have
> + * to keep an eye on the number of extents we're dirtying -- run too
> + * many in a single transaction and we'll exceed the transaction's
> + * reservation and crash the fs. Each record adds 12 bytes to the
> + * log (plus any key updates) so we'll conservatively assume 32 bytes
> + * per record. We must also leave space for btree splits on both ends
> + * of the range and space for the CUD and a new CUI.
> + */
> +#define XFS_REFCOUNT_ITEM_OVERHEAD 32
> +
> +static inline xfs_fileoff_t xfs_refcount_max_unmap(int log_res)
> +{
> + return (log_res * 3 / 4) / XFS_REFCOUNT_ITEM_OVERHEAD;
> +}
> +
> #endif /* __XFS_REFCOUNT_H__ */
> --
> 2.11.0
>
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