On Tue, Apr 26, 2022 at 05:51:58PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
>
> A long time ago, I added to XFS the ability to use deferred reference
> count operations as part of a transaction chain. This enabled us to
> avoid blowing out the transaction reservation when the blocks in a
> physical extent all had different reference counts because we could ask
> the deferred operation manager for a continuation, which would get us a
> clean transaction.
>
> The refcount code asks for a continuation when the number of refcount
> record updates reaches the point where we think that the transaction has
> logged enough full btree blocks due to refcount (and free space) btree
> shape changes and refcount record updates that we're in danger of
> overflowing the transaction.
>
> We did not previously count the EFIs logged to the refcount update
> transaction because the clamps on the length of a bunmap operation were
> sufficient to avoid overflowing the transaction reservation even in the
> worst case situation where every other block of the unmapped extent is
> shared.
>
> Unfortunately, the restrictions on bunmap length avoid failure in the
> worst case by imposing a maximum unmap length of ~3000 blocks, even for
> non-pathological cases. This seriously limits performance when freeing
> large extents.
>
> Therefore, track EFIs with the same counter as refcount record updates,
> and use that information as input into when we should ask for a
> continuation. This enables the next patch to drop the clumsy bunmap
> limitation.
>
> Depends: 27dada070d59 ("xfs: change the order in which child and parent defer ops ar finished")
> Depends: 74f4d6a1e065 ("xfs: only relog deferred intent items if free space in the log gets low")
> Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/libxfs/xfs_refcount.c | 5 ++---
> fs/xfs/libxfs/xfs_refcount.h | 13 ++++++++-----
> 2 files changed, 10 insertions(+), 8 deletions(-)
>
>
> diff --git a/fs/xfs/libxfs/xfs_refcount.c b/fs/xfs/libxfs/xfs_refcount.c
> index 327ba25e9e17..a07ebaecba73 100644
> --- a/fs/xfs/libxfs/xfs_refcount.c
> +++ b/fs/xfs/libxfs/xfs_refcount.c
> @@ -960,6 +960,7 @@ xfs_refcount_adjust_extents(
> * Either cover the hole (increment) or
> * delete the range (decrement).
> */
> + cur->bc_ag.refc.nr_ops++;
> if (tmp.rc_refcount) {
> error = xfs_refcount_insert(cur, &tmp,
> &found_tmp);
> @@ -970,7 +971,6 @@ xfs_refcount_adjust_extents(
> error = -EFSCORRUPTED;
> goto out_error;
> }
> - cur->bc_ag.refc.nr_ops++;
> } else {
> fsbno = XFS_AGB_TO_FSB(cur->bc_mp,
> cur->bc_ag.pag->pag_agno,
> @@ -1001,11 +1001,11 @@ xfs_refcount_adjust_extents(
> ext.rc_refcount += adj;
> trace_xfs_refcount_modify_extent(cur->bc_mp,
> cur->bc_ag.pag->pag_agno, &ext);
> + cur->bc_ag.refc.nr_ops++;
> if (ext.rc_refcount > 1) {
> error = xfs_refcount_update(cur, &ext);
> if (error)
> goto out_error;
> - cur->bc_ag.refc.nr_ops++;
> } else if (ext.rc_refcount == 1) {
> error = xfs_refcount_delete(cur, &found_rec);
> if (error)
> @@ -1014,7 +1014,6 @@ xfs_refcount_adjust_extents(
> error = -EFSCORRUPTED;
> goto out_error;
> }
> - cur->bc_ag.refc.nr_ops++;
> goto advloop;
> } else {
> fsbno = XFS_AGB_TO_FSB(cur->bc_mp,
> diff --git a/fs/xfs/libxfs/xfs_refcount.h b/fs/xfs/libxfs/xfs_refcount.h
> index 9eb01edbd89d..e8b322de7f3d 100644
> --- a/fs/xfs/libxfs/xfs_refcount.h
> +++ b/fs/xfs/libxfs/xfs_refcount.h
> @@ -67,14 +67,17 @@ extern int xfs_refcount_recover_cow_leftovers(struct xfs_mount *mp,
> * 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.
> + *
> + * Each EFI that we attach to the transaction is assumed to consume ~32 bytes.
> + * This is a low estimate for an EFI tracking a single extent (16 bytes for the
> + * EFI header, 16 for the extent, and 12 for the xlog op header), but the
> + * estimate is acceptable if there's more than one extent being freed.
> + * In the worst case of freeing every other block during a refcount decrease
> + * operation, we amortize the space used for one EFI log item across 16
> + * extents.
> */
> #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;
> -}
Oops, this helper should not have been deleted until the next patch.
I'll fix that before I send Dave a pull request.
--D
> -
> extern int xfs_refcount_has_record(struct xfs_btree_cur *cur,
> xfs_agblock_t bno, xfs_extlen_t len, bool *exists);
> union xfs_btree_rec;
>
