Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation

[Pavel Tatashin <pasha.tatashin@soleen.com>]

On Wed, Feb 3, 2021 at 8:23 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> On 1/25/21 7:47 PM, Pavel Tatashin wrote:
> > When pages are isolated in check_and_migrate_movable_pages() we skip
> > compound number of pages at a time. However, as Jason noted, it is
> > not necessary correct that pages[i] corresponds to the pages that
> > we skipped. This is because it is possible that the addresses in
> > this range had split_huge_pmd()/split_huge_pud(), and these functions
> > do not update the compound page metadata.
> >
> > The problem can be reproduced if something like this occurs:
> >
> > 1. User faulted huge pages.
> > 2. split_huge_pmd() was called for some reason
> > 3. User has unmapped some sub-pages in the range
> > 4. User tries to longterm pin the addresses.
> >
> > The resulting pages[i] might end-up having pages which are not compound
> > size page aligned.
> >
> > Fixes: aa712399c1e8 ("mm/gup: speed up check_and_migrate_cma_pages() on huge page")
> > Reported-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > ---
>
> [...]
>
> >               /*
> >                * If we get a page from the CMA zone, since we are going to
> >                * be pinning these entries, we might as well move them out
> > @@ -1599,8 +1596,6 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
> >                               }
> >                       }
> >               }
> > -
> > -             i += step;
> >       }
> >
>

Hi Joao,

> With this, longterm gup will 'regress' for hugetlbfs e.g. from ~6k -> 32k usecs when
> pinning a 16G hugetlb file.

Estimate or you actually measured?

>

> Splitting can only occur on THP right? If so, perhaps we could retain the @step increment

Yes, I do not think we can split HugePage, only THP.

> for compound pages but when !is_transparent_hugepage(head) or just PageHuge(head) like:
>
> +               if (!is_transparent_hugepage(head) && PageCompound(page))
> +                       i += (compound_nr(head) - (pages[i] - head));
>
> Or making specific to hugetlbfs:
>
> +               if (PageHuge(head))
> +                       i += (compound_nr(head) - (pages[i] - head));

Yes, this is reasonable optimization. I will submit a follow up patch
against linux-next.

Thank you,
Pasha