linux-kernel - Re: [PATCH v3 5/5] mm: defer second attempt at merge on mmap()

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Message-ID: <2e4d37bb-7333-4acc-92d8-c4ccc0d5ee13@lucifer.local>
Date: Fri, 25 Oct 2024 18:53:47 +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 v3 5/5] mm: defer second attempt at merge on mmap()

On Fri, Oct 25, 2024 at 07:40:43PM +0200, Vlastimil Babka wrote:
> On 10/25/24 14:26, 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.
> >
> > 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.
> >
> > Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> >  mm/vma.c | 55 ++++++++++++++-----------------------------------------
> >  1 file changed, 14 insertions(+), 41 deletions(-)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 7c690be67910..7194f9449c60 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;
> > @@ -2278,8 +2279,9 @@ static int __mmap_prepare(struct mmap_state *map, struct list_head *uf)
> >  	return 0;
> >  }
> >
> > +
> >  static int __mmap_new_file_vma(struct mmap_state *map,
> > -			       struct vm_area_struct **vmap, bool *mergedp)
> > +			       struct vm_area_struct **vmap)
>
> AFAICS **vmap could become *vma now

Ah yeah good spot you're right, will chase up on respin or do a fix-patch
next week!

>
> >  {
> >  	struct vma_iterator *vmi = map->vmi;
> >  	struct vm_area_struct *vma = *vmap;