linux-kernel - Re: [PATCH v2] xfs: check the remaining length of extent after roundup

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Message-ID: <20241108093228.1151891-1-alexjlzheng@tencent.com>
Date: Fri,  8 Nov 2024 17:32:28 +0800
From: Jinliang Zheng <alexjlzheng@...il.com>
To: djwong@...nel.org
Cc: alexjlzheng@...il.com,
	alexjlzheng@...cent.com,
	cem@...nel.org,
	chandanbabu@...nel.org,
	dchinner@...hat.com,
	linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org
Subject: Re: [PATCH v2] xfs: check the remaining length of extent after roundup

On Thu, 7 Nov 2024 12:05:39 -0800, Darrick J. Wong wrote:
> On Thu, Nov 07, 2024 at 04:40:44PM +0800, Jinliang Zheng wrote:
> > In xfs_alloc_compute_diff(), ensure that the remaining length of extent
> > still meets the wantlen requirements after newbno1 is rounded.
> 
> What problem are you observing?
> 
> --D

Thank you for your reply. :)

In fact, I haven't encountered any issues with this in production.

My starting point is I was wondering what will happen if
xfs_alloc_compute_diff()'s changes to bnew cause the extent's remaining
length to be less than args->len?

I wonder if this will happen? Am I missing some code to ensure this doesn't
happen?

If it will happen, I think we'd better check it out here; if it doesn't,
please ignore this patch.

Thank you again.
Jinliang Zheng :)

> 
> > Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> > ---
> > Changelog:
> > 
> > V2: Fix the error logic
> > 
> > V1: https://lore.kernel.org/linux-xfs/20241107070300.13535-1-alexjlzheng@tencent.com/#R
> > ---
> >  fs/xfs/libxfs/xfs_alloc.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > index 22bdbb3e9980..1d4cc75b7318 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.c
> > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > @@ -393,7 +393,8 @@ xfs_alloc_compute_diff(
> >  	 * grows in the short term.
> >  	 */
> >  	if (freebno >= wantbno || (userdata && freeend < wantend)) {
> > -		if ((newbno1 = roundup(freebno, alignment)) >= freeend)
> > +		newbno1 = roundup(freebno, alignment);
> > +		if (newbno1 >= freeend || newbno1 > freeend - wantlen)
> >  			newbno1 = NULLAGBLOCK;
> >  	} else if (freeend >= wantend && alignment > 1) {
> >  		newbno1 = roundup(wantbno, alignment);
> > @@ -414,6 +415,8 @@ xfs_alloc_compute_diff(
> >  				newbno1 = newbno2;
> >  		} else if (newbno2 != NULLAGBLOCK)
> >  			newbno1 = newbno2;
> > +		if (newbno1 > freeend - wantlen)
> > +			newbno1 = NULLAGBLOCK;
> >  	} else if (freeend >= wantend) {
> >  		newbno1 = wantbno;
> >  	} else if (alignment > 1) {
> > -- 
> > 2.41.1
> > 
> >