Re: [PATCH 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free

[Lance Yang]

On Mon, Feb 26, 2024 at 9:04 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 26.02.24 13:57, Ryan Roberts wrote:
> > On 26/02/2024 08:35, Lance Yang wrote:
> >> Hey Fengwei,
> >>
> >> Thanks for taking time to review!
> >>
> >>> On Mon, Feb 26, 2024 at 10:38 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
> >>>> On Sun, Feb 25, 2024 at 8:32 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >> [...]
> >>>> --- a/mm/madvise.c
> >>>> +++ b/mm/madvise.c
> >>>> @@ -676,11 +676,43 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >>>>                 */
> >>>>                if (folio_test_large(folio)) {
> >>>>                        int err;
> >>>> +                     unsigned long next_addr, align;
> >>>>
> >>>> -                     if (folio_estimated_sharers(folio) != 1)
> >>>> -                             break;
> >>>> -                     if (!folio_trylock(folio))
> >>>> -                             break;
> >>>> +                     if (folio_estimated_sharers(folio) != 1 ||
> >>>> +                         !folio_trylock(folio))
> >>>> +                             goto skip_large_folio;
> >>>> +
> >>>> +                     align = folio_nr_pages(folio) * PAGE_SIZE;
> >>>> +                     next_addr = ALIGN_DOWN(addr + align, align);
> >>> There is a possible corner case:
> >>> If there is a cow folio associated with this folio and the cow folio
> >>> has smaller size than this folio for whatever reason, this change can't
> >>> handle it correctly.
> >>
> >> Thanks for pointing that out; it's very helpful to me!
> >> I made some changes. Could you please check if this corner case is now resolved?
> >>
> >> As a diff against this patch.
> >>
> >> diff --git a/mm/madvise.c b/mm/madvise.c
> >> index bcbf56595a2e..c7aacc9f9536 100644
> >> --- a/mm/madvise.c
> >> +++ b/mm/madvise.c
> >> @@ -686,10 +686,12 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >>                      next_addr = ALIGN_DOWN(addr + align, align);
> >>
> >>                      /*
> >> -                     * If we mark only the subpages as lazyfree,
> >> -                     * split the large folio.
> >> +                     * If we mark only the subpages as lazyfree, or
> >> +                     * if there is a cow folio associated with this folio,
> >> +                     * then split the large folio.
> >>                       */
> >> -                    if (next_addr > end || next_addr - addr != align)
> >> +                    if (next_addr > end || next_addr - addr != align ||
> >> +                        folio_total_mapcount(folio) != folio_nr_pages(folio))
> >
> > I still don't think this is correct. I think you were previously assuming that
> > if you see a page from a large folio then the whole large folio should be
> > contiguously mapped? This new check doesn't validate that assumption reliably;
> > you need to iterate through every pte to generate a batch, like David does in
> > folio_pte_batch() for this to be safe.
> >
> > An example of when this check is insufficient; let's say you have a 4 page anon
> > folio mapped contiguously in a process (total_mapcount=4). The process is forked
> > (total_mapcount=8). Then each process munmaps the second 2 pages
> > (total_mapcount=4). In place of the munmapped 2 pages, 2 new pages are mapped.
> > Then call madvise. It's probably even easier to trigger for file-backed memory
> > (I think this code path is used for both file and anon?)
>
> What would work here is using folio_pte_batch() to get how many PTEs are
> mapped *here*, then comparing the the batch size to folio_nr_pages(). If
> both match, we are mapping all subpages.

Thanks! I'll use folio_pte_batch() here in v2.

Best,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>