linux-kernel - Re: [PATCH v2 8/8] mm: defer second attempt at merge on mmap()

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Message-ID: <e76fb4dc-3134-46fa-ab51-f28e542e1880@lucifer.local>
Date: Fri, 25 Oct 2024 11:20:31 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2 8/8] mm: defer second attempt at merge on mmap()

On Fri, Oct 25, 2024 at 11:43:20AM +0200, Vlastimil Babka wrote:
> On 10/23/24 22:38, Lorenzo Stoakes wrote:
> > Rather than trying to merge again when ostensibly allocating a new VMA,
> > instead defer until the VMA is added and attempt to merge the existing
> > range.
> >
> > This way we have no complicated unwinding logic midway through the process
> > of mapping the VMA.
> >
> > In addition this removes limitations on the VMA not being able to be the
> > first in the virtual memory address space which was previously implicitly
> > required.
> >
> > It also performs this merge after the final flag adjustments are performed,
> > something that was not done previously and thus might have prevented
> > possibly valid merges in the past.
> >
> > In theory, for this very same reason, we should unconditionally attempt
> > merge here, however this is likely to have a performance impact so it is
> > better to avoid this given the unlikely outcome of a merge.
>
> Maybe just expand the cases where we set map->retry_merge, i.e. in case the
> final flag adjustments do anything?
>
> > The vmg state will already have been reset by the first attempt at a merge
> > so we only need to reset the iterator, set the vma and flags and try again.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks (and for 7/8!)

>
> A nit:
>
> > ---
> >  mm/vma.c | 75 ++++++++++++++++++++------------------------------------
> >  1 file changed, 26 insertions(+), 49 deletions(-)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 065f5e1f65be..c493ecebf394 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -19,6 +19,7 @@ struct mmap_state {
> >  	struct file *file;
> >
> >  	unsigned long charged;
> > +	bool retry_merge;
> >
> >  	struct vm_area_struct *prev;
> >  	struct vm_area_struct *next;
> > @@ -2280,9 +2281,9 @@ static int __mmap_prepare(struct mmap_state *map, struct vma_merge_struct *vmg,
> >  	return 0;
> >  }
> >
> > +
> >  static int __mmap_new_file_vma(struct mmap_state *map,
> > -			       struct vma_merge_struct *vmg,
> > -			       struct vm_area_struct **vmap, bool *mergedp)
> > +			       struct vm_area_struct **vmap)
> >  {
> >  	struct vma_iterator *vmi = map->vmi;
> >  	struct vm_area_struct *vma = *vmap;
> > @@ -2311,37 +2312,11 @@ static int __mmap_new_file_vma(struct mmap_state *map,
> >  			!(map->flags & VM_MAYWRITE) &&
> >  			(vma->vm_flags & VM_MAYWRITE));
> >
> > -	vma_iter_config(vmi, map->addr, map->end);
> > -	/*
> > -	 * If flags changed after mmap_file(), we should try merge
> > -	 * vma again as we may succeed this time.
> > -	 */
> > -	if (unlikely(map->flags != vma->vm_flags && map->prev)) {
> > -		struct vm_area_struct *merge;
> > -
> > -		vmg->flags = vma->vm_flags;
> > -		/* If this fails, state is reset ready for a reattempt. */
> > -		merge = vma_merge_new_range(vmg);
> > -
> > -		if (merge) {
> > -			/*
> > -			 * ->mmap() can change vma->vm_file and fput
> > -			 * the original file. So fput the vma->vm_file
> > -			 * here or we would add an extra fput for file
> > -			 * and cause general protection fault
> > -			 * ultimately.
> > -			 */
> > -			fput(vma->vm_file);
> > -			vm_area_free(vma);
> > -			vma = merge;
> > -			*mergedp = true;
> > -		} else {
> > -			vma_iter_config(vmi, map->addr, map->end);
> > -		}
> > -	}
> > +	/* If the flags change (and are mergeable), let's retry later. */
> > +	map->retry_merge = vma->vm_flags != map->flags && !(vma->vm_flags & VM_SPECIAL);
> >
> > +	vma_iter_config(vmi, map->addr, map->end);
>
> Do we need this still? __mmap_new_vma() did that and nothing changed since
> in the non-error case, AFAICS?

You're right, this change really highlights that, will remove thanks!

>
> >  	map->flags = vma->vm_flags;
> > -	*vmap = vma;
> >  	return 0;
> >  }
> >