linux-kernel - Re: [PATCH v3 10/16] mm/mmap: Reposition vma iterator in mmap

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Message-ID: <c7vcig3ee4n3zpfev3tbv5epcsdeue5it3joi22iwob5vx24sg@ypl6ienxns76>
Date: Fri, 5 Jul 2024 16:56:34 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lstoakes@...il.com>,
        Matthew Wilcox <willy@...radead.org>, sidhartha.kumar@...cle.com,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Bert Karwatzki <spasswolf@....de>, Jiri Olsa <olsajiri@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 10/16] mm/mmap: Reposition vma iterator in
 mmap_region()

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [240705 16:18]:
> On Thu, Jul 04, 2024 at 02:27:12PM GMT, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> >
> > Instead of moving (or leaving) the vma iterator pointing at the previous
> > vma, leave it pointing at the insert location.  Pointing the vma
> > iterator at the insert location allows for a cleaner walk of the vma
> > tree for MAP_FIXED and the no expansion cases.
> 
> I mean, it's funny I litearlly just asked why it was being left pointing at
> prev and I guess this answers that basically :)
> 
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > ---
> >  mm/mmap.c | 28 ++++++++++++++--------------
> >  1 file changed, 14 insertions(+), 14 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index f5b33de4e717..ecf55d32e804 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2963,11 +2963,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  		vms_complete_munmap_vmas(&vms, &mas_detach);
> >  		next = vms.next;
> >  		prev = vms.prev;
> > -		vma_prev(&vmi);
> >  		vma = NULL;
> >  	} else {
> >  		next = vma_next(&vmi);
> >  		prev = vma_prev(&vmi);
> > +		if (prev)
> > +			vma_iter_next_range(&vmi);
> >  	}
> >
> >  	/*
> > @@ -2980,11 +2981,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  		vm_flags |= VM_ACCOUNT;
> >  	}
> >
> > -	if (vm_flags & VM_SPECIAL) {
> > -		if (prev)
> > -			vma_iter_next_range(&vmi);
> > +	if (vm_flags & VM_SPECIAL)
> >  		goto cannot_expand;
> > -	}
> >
> >  	/* Attempt to expand an old mapping */
> >  	/* Check next */
> > @@ -3005,19 +3003,21 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  		merge_start = prev->vm_start;
> >  		vma = prev;
> >  		vm_pgoff = prev->vm_pgoff;
> > -	} else if (prev) {
> > -		vma_iter_next_range(&vmi);
> > +		vma_prev(&vmi);
> 
> Why not vma_iter_prev_range()?

In any situation where we arrive at prev being able to merge with the
current, the prev->end == addr and we are already pointing at the gap
with addr.  So vma_iter_prev_range() and vma_prev() will do the same
thing here.

I thought a lot about this (and created a small test application since
it is so difficult to test vma merging right now..).  If we do not clear
the gap, it is possible we will be pointing at the first vma in the
range, and not addr at all when we call vms_gather_munmap_vmas(), but
the loop at the end of the gather function ensures the vmi is at the
correct address (vms->start is in the range). So the result is that we
consistently point to the correct location and so these two calls will
do the same thing.

> 
> >  	}
> >
> > -	/* Actually expand, if possible */
> > -	if (vma &&
> > -	    !vma_expand(&vmi, vma, merge_start, merge_end, vm_pgoff, next)) {
> > -		khugepaged_enter_vma(vma, vm_flags);
> > -		goto expanded;
> > +	if (vma) {
> > +		/* Actually expand, if possible */
> > +		if (!vma_expand(&vmi, vma, merge_start, merge_end, vm_pgoff, next)) {
> > +			khugepaged_enter_vma(vma, vm_flags);
> > +			goto expanded;
> > +		}
> > +
> > +		/* If the expand fails, then reposition the vma iterator */
> > +		if (unlikely(vma == prev))
> > +			vma_iter_set(&vmi, addr);
> >  	}
> >
> > -	if (vma == prev)
> > -		vma_iter_set(&vmi, addr);
> >  cannot_expand:
> >
> >  	/*
> > --
> > 2.43.0
> >
> Looks good to me,
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Thanks!