linux-kernel - Re: [PATCH 4/5] mm: make vmg->target consistent and further simplify commit

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Message-ID: <0842ba03-0b0c-4372-9b1f-33b966944c10@lucifer.local>
Date: Wed, 29 Jan 2025 14:51:54 +0000
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-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] mm: make vmg->target consistent and further simplify
 commit_merge()

On Wed, Jan 29, 2025 at 03:45:28PM +0100, Vlastimil Babka wrote:
> On 1/27/25 16:50, Lorenzo Stoakes wrote:
> > It is confusing for vmg->target to sometimes be the target merged VMA and
> > in one case not.
> >
> > Fix this by having commit_merge() use its awareness of the
> > __VMG_FLAG_ADJUST_NEXT_START case to know that it is manipulating a
> > separate vma, abstracted in the 'vma' local variable.
> >
> > Place removal and adjust VMA determination logic into
> > init_multi_vma_prep(), as the flags give us enough information to do so,
> > and since this is the function that sets up the vma_prepare struct it makes
> > sense to do so here.
> >
> > Doing this significantly simplifies commit_merge(), allowing us to
> > eliminate the 'merge_target' handling, initialise the VMA iterator in a
> > more sensible place and simply return vmg->target consistently.
> >
> > This also allows us to simplify setting vmg->target in
> > vma_merge_existing_range() since we are then left only with two cases -
> > merge left (or both) where the target is vmg->prev or merge right in which
> > the target is vmg->next.
> >
> > This makes it easy for somebody reading the code to know what VMA will
> > actually be the one returned and merged into and removes a great deal of
> > the confusing 'adjust' nonsense.
> >
> > This patch has no change in functional behaviour.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

>
>
> > @@ -906,7 +911,6 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
> >  			 * merged VMA is NOT vmg->target, but rather vmg->next.
> >  			 */
>
> Is this comment now also obsolete?
>

It will be in 5/5 where it gets deleted.

> >  			vmg->merge_flags |= __VMG_FLAG_ADJUST_NEXT_START;
> > -			vmg->target = middle;
> >  			vmg->start = middle->vm_start;
> >  			vmg->end = start;
> >  			vmg->pgoff = middle->vm_pgoff;
> > diff --git a/mm/vma.h b/mm/vma.h
> > index ddf567359880..5be43e2bba3f 100644
> > --- a/mm/vma.h
> > +++ b/mm/vma.h
> > @@ -113,11 +113,7 @@ struct vma_merge_struct {
> >  	struct vm_area_struct *prev;
> >  	struct vm_area_struct *middle;
> >  	struct vm_area_struct *next;
> > -	/*
> > -	 * This is the VMA we ultimately target to become the merged VMA, except
> > -	 * for the one exception of merge right, shrink next (for details of
> > -	 * this scenario see vma_merge_existing_range()).
> > -	 */
> > +	/* This is the VMA we ultimately target to become the merged VMA. */
> >  	struct vm_area_struct *target;
> >  	/*
> >  	 * Initially, the start, end, pgoff fields are provided by the caller
>