Re: [PATCH 2/2] mm: pvmw: check possible huge PMD map by transhuge_vma_suitable()

[Yang Shi <shy828301@gmail.com>]

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Andrew Morton <akpm@linux-foundation.org>于2022年5月12日 周四下午4:39写道：

> On Thu, 12 May 2022 10:45:51 -0700 Yang Shi <shy828301@gmail.com> 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.
> >
> > ...
> >
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -237,13 +237,14 @@ bool page_vma_mapped_walk(struct
> page_vma_mapped_walk *pvmw)
> >                       spin_unlock(pvmw->ptl);
> >                       pvmw->ptl = NULL;
> >               } else if (!pmd_present(pmde)) {
> > +                     unsigned long haddr = pvmw->address &
> HPAGE_PMD_MASK;
>
> This hits
>
>         #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
>
> when CONFIG_TRANSPARENT_HUGEPAGE=n (x86_64 allnoconfig).


Thanks for catching this. I think the best way is to round the address in
transhuge_vma_suitable() which is protected by the config.

Will prepare v2 soon.


>
>

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