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Message-ID: <Ygxbge2l2jde8Xcp@B-P7TQMD6M-0146.local>
Date: Wed, 16 Feb 2022 10:03:45 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: xfs <linux-xfs@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] xfs: introduce xfs_bremapi_from_cowfork()
Hi Darrick,
On Tue, Feb 15, 2022 at 05:24:33PM -0800, Darrick J. Wong wrote:
> On Wed, Feb 09, 2022 at 03:36:55PM +0800, Gao Xiang wrote:
> > Previously, xfs_reflink_end_cow_extent() will unconditionally unmap
> > the corresponding old extent and remap an extent from COW fork.
> > However, it seems somewhat ineffective since the old bmbt records can
> > be directly updated for many cases instead.
> >
> > This patch uses introduced xfs_bmap_update_extent_real() in the
> > previous patch for most extent inclusive cases or it will fall back
> > to the old way if such replacement is not possible.
> >
> > Actually, we're planing to use a modified alway-cow like atomic write
> > approach internally, therefore it'd be nice to do some optimization
> > to reduce some metadata overhead.
> >
> > Signed-off-by: Gao Xiang <hsiangkao@...ux.alibaba.com>
> > ---
> > fs/xfs/libxfs/xfs_bmap.c | 117 ++++++++++++++++++++++++++++++++++++---
> > fs/xfs/libxfs/xfs_bmap.h | 3 +
> > fs/xfs/xfs_reflink.c | 19 +------
> > 3 files changed, 112 insertions(+), 27 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index a10476dee701..0e132f811f7a 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -5880,6 +5880,114 @@ xfs_bmap_collapse_extents(
> > return error;
> > }
> >
> > +/* Deferred mapping is only for real extents in the data fork. */
> > +static bool
> > +xfs_bmap_is_update_needed(
> > + struct xfs_bmbt_irec *bmap)
> > +{
> > + return bmap->br_startblock != HOLESTARTBLOCK &&
> > + bmap->br_startblock != DELAYSTARTBLOCK;
> > +}
> > +
> > +/* del is an extent from COW fork */
> > +int
> > +xfs_bremapi_from_cowfork(
> > + struct xfs_trans *tp,
> > + struct xfs_inode *ip,
> > + struct xfs_bmbt_irec *icow)
> > +{
> > + int error;
> > + xfs_filblks_t rlen;
> > +
> > + /* Use the old (unmap-remap) way for real-time inodes instead */
> > + if (!XFS_IS_REALTIME_INODE(ip) && xfs_bmap_is_update_needed(icow)) {
>
> When would be be remapping a realtime file with a COW fork?
Sorry, I missed this part, this is a necessary check.
Thanks for pointing out this.
>
> > + xfs_fileoff_t start, end, max_len;
> > + struct xfs_bmbt_irec got;
> > + struct xfs_iext_cursor icur;
> > + struct xfs_btree_cur *cur = NULL;
> > + struct xfs_ifork *ifp = XFS_IFORK_PTR(ip, XFS_DATA_FORK);
> > + int logflags = 0;
> > +
> > + error = xfs_iread_extents(tp, ip, XFS_DATA_FORK);
> > + if (error)
> > + return error;
> > +
> > + max_len = xfs_refcount_max_unmap(tp->t_log_res);
> > + if (max_len < icow->br_blockcount) {
> > + icow->br_startoff += icow->br_blockcount - max_len;
> > + icow->br_startblock += icow->br_blockcount - max_len;
> > + icow->br_blockcount = max_len;
> > + }
> > +
> > + end = icow->br_startoff + icow->br_blockcount;
> > + if (!xfs_iext_count(ifp) || !xfs_iext_lookup_extent_before(ip,
> > + ifp, &end, &icur, &got) ||
> > + isnullstartblock(got.br_startblock) ||
> > + icow->br_startoff + icow->br_blockcount > got.br_startoff +
> > + got.br_blockcount) {
> > + error = -EAGAIN;
> > + } else {
> > + end = icow->br_startoff + icow->br_blockcount;
> > + start = XFS_FILEOFF_MAX(icow->br_startoff,
> > + got.br_startoff);
> > + ASSERT(start < end);
> > +
> > + /* Trim the extent to what we need */
> > + xfs_trim_extent(icow, start, end - start);
> > + xfs_trim_extent(&got, start, end - start);
> > +
> > + if (ifp->if_format == XFS_DINODE_FMT_BTREE) {
> > + cur = xfs_bmbt_init_cursor(tp->t_mountp, tp, ip,
> > + XFS_DATA_FORK);
> > + cur->bc_ino.flags = 0;
> > + }
> > +
> > + /*
> > + * Free the CoW orphan record (it should be done here
> > + * before updating extent due to rmapbt update)
> > + */
> > + xfs_refcount_free_cow_extent(tp, icow->br_startblock,
> > + icow->br_blockcount);
> > +
> > + xfs_bmap_update_extent_real(tp, ip, XFS_DATA_FORK,
> > + &icur, &cur, icow, &logflags, false);
>
> Hmm... are you directly updating the data fork mapping record from the
> COW fork mapping record? Is the performance advantage you mentioned
> earlier a result of this code no longer logging a bmap map intent item
> and reducing the transaction roll count by one?
Yes, roughly I think it reduces bmap again overhead.
Thanks,
Gao Xiang
>
> --D
>
> > +
> > + /* Free previous referenced space */
> > + xfs_refcount_decrease_extent(tp, &got);
> > +
> > + trace_xfs_reflink_cow_remap(ip, icow);
> > + error = 0;
> > + }
> > + if (cur)
> > + xfs_btree_del_cursor(cur, 0);
> > + if (logflags)
> > + xfs_trans_log_inode(tp, ip, logflags);
> > + if (!error)
> > + return 0;
> > + }
> > +
> > + rlen = icow->br_blockcount;
> > + error = __xfs_bunmapi(tp, ip, icow->br_startoff, &rlen, 0, 1);
> > + if (error)
> > + return error;
> > +
> > + /* Trim the extent to whatever got unmapped. */
> > + xfs_trim_extent(icow, icow->br_startoff + rlen,
> > + icow->br_blockcount - rlen);
> > + /* Free the CoW orphan record. */
> > + xfs_refcount_free_cow_extent(tp, icow->br_startblock,
> > + icow->br_blockcount);
> > +
> > + /* Map the new blocks into the data fork. */
> > + xfs_bmap_map_extent(tp, ip, icow);
> > +
> > + /* Charge this new data fork mapping to the on-disk quota. */
> > + xfs_trans_mod_dquot_byino(tp, ip, XFS_TRANS_DQ_DELBCOUNT,
> > + (long)icow->br_blockcount);
> > + trace_xfs_reflink_cow_remap(ip, icow);
> > + return 0;
> > +}
> > +
> > /* Make sure we won't be right-shifting an extent past the maximum bound. */
> > int
> > xfs_bmap_can_insert_extents(
> > @@ -6123,15 +6231,6 @@ xfs_bmap_split_extent(
> > return error;
> > }
> >
> > -/* Deferred mapping is only for real extents in the data fork. */
> > -static bool
> > -xfs_bmap_is_update_needed(
> > - struct xfs_bmbt_irec *bmap)
> > -{
> > - return bmap->br_startblock != HOLESTARTBLOCK &&
> > - bmap->br_startblock != DELAYSTARTBLOCK;
> > -}
> > -
> > /* Record a bmap intent. */
> > static int
> > __xfs_bmap_add(
> > diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
> > index c52ff94786e2..9da1cff41c1c 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.h
> > +++ b/fs/xfs/libxfs/xfs_bmap.h
> > @@ -220,6 +220,9 @@ int xfs_bmap_update_extent_real(struct xfs_trans *tp,
> > struct xfs_inode *ip, int whichfork,
> > struct xfs_iext_cursor *icur, struct xfs_btree_cur **curp,
> > struct xfs_bmbt_irec *new, int *logflagsp, bool convert);
> > +int
> > +xfs_bremapi_from_cowfork(struct xfs_trans *tp, struct xfs_inode *ip,
> > + struct xfs_bmbt_irec *icow);
> >
> > enum xfs_bmap_intent_type {
> > XFS_BMAP_MAP = 1,
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index 276387a6a85d..75bd2e03cd5b 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -590,7 +590,6 @@ xfs_reflink_end_cow_extent(
> > struct xfs_mount *mp = ip->i_mount;
> > struct xfs_trans *tp;
> > struct xfs_ifork *ifp = XFS_IFORK_PTR(ip, XFS_COW_FORK);
> > - xfs_filblks_t rlen;
> > unsigned int resblks;
> > int error;
> >
> > @@ -651,26 +650,10 @@ xfs_reflink_end_cow_extent(
> > goto out_cancel;
> > }
> >
> > - /* Unmap the old blocks in the data fork. */
> > - rlen = del.br_blockcount;
> > - error = __xfs_bunmapi(tp, ip, del.br_startoff, &rlen, 0, 1);
> > + error = xfs_bremapi_from_cowfork(tp, ip, &del);
> > if (error)
> > goto out_cancel;
> >
> > - /* Trim the extent to whatever got unmapped. */
> > - xfs_trim_extent(&del, del.br_startoff + rlen, del.br_blockcount - rlen);
> > - trace_xfs_reflink_cow_remap(ip, &del);
> > -
> > - /* Free the CoW orphan record. */
> > - xfs_refcount_free_cow_extent(tp, del.br_startblock, del.br_blockcount);
> > -
> > - /* Map the new blocks into the data fork. */
> > - xfs_bmap_map_extent(tp, ip, &del);
> > -
> > - /* Charge this new data fork mapping to the on-disk quota. */
> > - xfs_trans_mod_dquot_byino(tp, ip, XFS_TRANS_DQ_DELBCOUNT,
> > - (long)del.br_blockcount);
> > -
> > /* Remove the mapping from the CoW fork. */
> > xfs_bmap_del_extent_cow(ip, &icur, &got, &del);
> >
> > --
> > 2.24.4
> >
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