linux-kernel - Re: [RESEND PATCH v2] xfs: fix the entry condition of exact EOF block allocation optimization

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Message-ID: <20241205121802.1232223-1-alexjlzheng@tencent.com>
Date: Thu,  5 Dec 2024 20:18:02 +0800
From: Jinliang Zheng <alexjlzheng@...il.com>
To: david@...morbit.com
Cc: alexjlzheng@...il.com,
	alexjlzheng@...cent.com,
	cem@...nel.org,
	chandanbabu@...nel.org,
	dchinner@...hat.com,
	djwong@...nel.org,
	hch@...radead.org,
	linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org
Subject: Re: [RESEND PATCH v2] xfs: fix the entry condition of exact EOF block allocation optimization

On Wed, 4 Dec 2024 07:40:20 +1100, Dave Chinner wrote:
> On Sat, Nov 31, 2024 at 07:11:32PM +0800, Jinliang Zheng wrote:
> > When we call create(), lseek() and write() sequentially, offset != 0
> > cannot be used as a judgment condition for whether the file already
> > has extents.
> > 
> > Furthermore, when xfs_bmap_adjacent() has not given a better blkno,
> > it is not necessary to use exact EOF block allocation.
> > 
> > Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> > ---
> > Changelog:
> > - V2: Fix the entry condition
> > - V1: https://lore.kernel.org/linux-xfs/ZyFJm7xg7Msd6eVr@dread.disaster.area/T/#t
> > ---
> >  fs/xfs/libxfs/xfs_bmap.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 36dd08d13293..c1e5372b6b2e 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -3531,12 +3531,14 @@ xfs_bmap_btalloc_at_eof(
> >  	int			error;
> >  
> >  	/*
> > -	 * If there are already extents in the file, try an exact EOF block
> > -	 * allocation to extend the file as a contiguous extent. If that fails,
> > -	 * or it's the first allocation in a file, just try for a stripe aligned
> > -	 * allocation.
> > +	 * If there are already extents in the file, and xfs_bmap_adjacent() has
> > +	 * given a better blkno, try an exact EOF block allocation to extend the
> > +	 * file as a contiguous extent. If that fails, or it's the first
> > +	 * allocation in a file, just try for a stripe aligned allocation.
> >  	 */
> > -	if (ap->offset) {
> > +	if (ap->prev.br_startoff != NULLFILEOFF &&
> > +	     !isnullstartblock(ap->prev.br_startblock) &&
> > +	     xfs_bmap_adjacent_valid(ap, ap->blkno, ap->prev.br_startblock)) {
> 
> There's no need for calling xfs_bmap_adjacent_valid() here -
> we know that ap->blkno is valid because the
> bounds checking has already been done by xfs_bmap_adjacent().

I'm sorry that I didn't express it clearly, what I meant here is: if we want
to extend the file as a contiguous extent, then ap->blkno must be a better
choice given by xfs_bmap_adjacent() than other default values.

/*
 * If allocating at eof, and there's a previous real block,
 * try to use its last block as our starting point.
 */
if (ap->eof && ap->prev.br_startoff != NULLFILEOFF &&
    !isnullstartblock(ap->prev.br_startblock) &&
    xfs_bmap_adjacent_valid(ap,
		ap->prev.br_startblock + ap->prev.br_blockcount,
		ap->prev.br_startblock)) {
	ap->blkno = ap->prev.br_startblock + ap->prev.br_blockcount; <--- better A
	/*
	 * Adjust for the gap between prevp and us.
	 */
	adjust = ap->offset -
		(ap->prev.br_startoff + ap->prev.br_blockcount);
	if (adjust && xfs_bmap_adjacent_valid(ap, ap->blkno + adjust,
			ap->prev.br_startblock))
		ap->blkno += adjust;                                 <--- better B
	return true;
}

Only when we reach 'better A' or 'better B' of xfs_bmap_adjacent() above, it
is worth trying to use xfs_alloc_vextent_EXACT_bno(). Otherwise, NEAR is
more suitable than EXACT.

Therefore, we need xfs_bmap_adjacent() to determine whether xfs_bmap_adjacent()
has indeed modified ap->blkno.

> 
> Actually, for another patch, the bounds checking in
> xfs_bmap_adjacent_valid() is incorrect. What happens if the last AG
> is a runt? i.e. it open codes xfs_verify_fsbno() and gets it wrong.

For general scenarios, I agree.

But here, the parameters x and y of xfs_bmap_adjacent_valid() are both derived
from ap->prev. Is it possible that it exceeds mp->m_sb.sb_agcount or
mp->m_sb.sb_agblocks?

Thank you :)
Jinliang Zheng

> 
> -Dave.
> -- 
> Dave Chinner
> david@...morbit.com