linux-kernel - Re: [PATCH 2/2] mm: pvmw: check possible huge PMD map by transhuge_vma

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Date:   Thu, 12 May 2022 20:51:26 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: pvmw: check possible huge PMD map by transhuge_vma_suitable()

On Thu, May 12, 2022 at 8:30 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, May 12, 2022 at 10:45:51AM -0700, Yang Shi wrote:
> > IIUC PVMW checks if the vma is possibly huge PMD mapped by
> > transparent_hugepage_active() and "pvmw->nr_pages >= HPAGE_PMD_NR".
> >
> > Actually pvmw->nr_pages is returned by compound_nr() or
> > folio_nr_pages(), so the page should be THP as long as "pvmw->nr_pages
> > >= HPAGE_PMD_NR".  And it is guaranteed THP is allocated for valid VMA
> > in the first place.  But it may be not PMD mapped if the VMA is file
> > VMA and it is not properly aligned.  The transhuge_vma_suitable()
> > is used to do such check, so replace transparent_hugepage_active() to
> > it, which is too heavy and overkilling.
> >
> > Fixes: 2aff7a4755be ("mm: Convert page_vma_mapped_walk to work on PFNs")
>
> I think Fixes is a bit much.  There's no bug being fixed here.  This is
> just an optimisation.  Is it an important optimisation?  We could put a

Yeah, it is just an optimization, will remove the fix tag.

I'm trying to do some cleanup for all the transhuge_page_* checks
suggested by Vlastimil. I should be able to kill
transparent_hugepage_active() by replacing it with
transhuge_vma_suitable() here.

> bool into page_vma_mapped_walk() so we only have to ask the page whether
> it's PMD-mappable once per walk rather than for each VMA.

The page may be PMD-mappable for one VMA, but not for the other VMA.

>