linux-kernel - Re: [PATCH v2] mm/mmap: return early if it can't merge in vma

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Message-ID: <f3847dd7-5564-4d7e-951e-1a9d8f55fb78@lucifer.local>
Date: Wed, 21 Feb 2024 20:41:39 +0000
From: Lorenzo Stoakes <lstoakes@...il.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Yajun Deng <yajun.deng@...ux.dev>, akpm@...ux-foundation.org,
	vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/mmap: return early if it can't merge in vma_merge()

On Wed, Feb 21, 2024 at 10:38:27AM -0500, Liam R. Howlett wrote:
> * Yajun Deng <yajun.deng@...ux.dev> [240221 04:15]:
> > In most cases, the range of the area is valid. But in do_mprotect_pkey(),
> > the minimum value of end and vma->vm_end is passed to mprotect_fixup().
> > This will lead to the end is less than the end of prev.
> >
> > In this case, the curr will be NULL, but the next will be equal to the
> > prev. So it will attempt to merge before, the vm_pgoff check will cause
> > this case to fail.
> >
> > To avoid the process described above and reduce unnecessary operations.
> > Add a check to immediately return NULL if the end is less than the end of
> > prev.
>
> If it's only one caller, could we stop that caller instead of checking
> an almost never case for all callers?  Would this better fit in
> vma_modify()?  Although that's not just for this caller at this point.
> Maybe there isn't a good place?

I definitely agree with Liam that this should not be in vma_merge(), as
it's not going to be relevant to _most_ callers.

I am not sure vma_modify() is much better, this would be the only early
exit check in that function and makes what is very simple and
straightforward now more confusing.

And I think this is the crux of it - it's confusing that we special case
this one particular non-merge scenario, but no others (all of which we then
deem ok to be caught by the usual rules).

I think it's simpler (and more efficient) to just keep things the way they
are.

>
> Or are there other reasons this may happen and is better done in this
> function?
>
> Often, this is called the "punch a hole" scenario; where an operation
> creates two entries from the old data and either leaves an empty space
> or fills the space with a new VMA.
>
> >
> > Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> > ---
> > v2: remove the case label.
> > v1: https://lore.kernel.org/all/20240218085028.3294332-1-yajun.deng@linux.dev/
> > ---
> >  mm/mmap.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 0fccd23f056e..7668854d2246 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -890,6 +890,9 @@ static struct vm_area_struct
> >  	if (vm_flags & VM_SPECIAL)
> >  		return NULL;
> >
> > +	if (prev && end < prev->vm_end)
> > +		return NULL;
> > +
> >  	/* Does the input range span an existing VMA? (cases 5 - 8) */
> >  	curr = find_vma_intersection(mm, prev ? prev->vm_end : 0, end);
> >
> > --
> > 2.25.1
> >

So overall I don't think this check makes much sense anywhere.

I think a better solution would be to prevent it happening _at source_ if
you can find a logical way of doing so.

I do plan to do some cleanup passes over this stuff once again so maybe I
can figure something out that better handles non-merge scenarios.

This is a great find though overall even if a patch doesn't make sense
Yajun, thanks for this, it's really made me think about this case (+ others
like it) :)