Re: [RFC PATCH 07/14] mm/khugepaged: add vm_flags_ignore to hugepage_vma_revalidate_pmd_count()

[Yang Shi <shy828301@gmail.com>]

On Wed, Mar 9, 2022 at 5:10 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> On Wed, Mar 9, 2022 at 4:41 PM Yang Shi <shy828301@gmail.com> wrote:
> >
> > On Wed, Mar 9, 2022 at 4:01 PM Zach O'Keefe <zokeefe@google.com> wrote:
> > >
> > > > On Tue, Mar 8, 2022 at 1:35 PM Zach O'Keefe <zokeefe@google.com> wrote:
> > > > >
> > > > > In madvise collapse context, we optionally want to be able to ignore
> > > > > advice from MADV_NOHUGEPAGE-marked regions.
> > > >
> > > > Could you please elaborate why this usecase is valid? Typically
> > > > MADV_NOHUGEPAGE is set when the users really don't want to have THP
> > > > for this area. So it doesn't make too much sense to ignore it IMHO.
> > > >
> > >
> > > Hey Yang, thanks for taking time to review and comment.
> > >
> > > Semantically, the way I see it, is that MADV_NOHUGEPAGE is a way for
> > > the user to say "I don't want hugepages here", so that the kernel
> > > knows not to do so when faulting memory, and khugepaged can stay away.
> > > However, in MADV_COLLAPSE, the user is explicitly requesting this be
> > > backed by hugepages - so presumably that is exactly what they want.
> > >
> > > IOW, if the user didn't want this memory to be backed by hugepages,
> > > they wouldn't be MADV_COLLAPSE'ing it. If there was a range of memory
> > > the user wanted collapsed, but that had some sub-areas marked
> > > MADV_NOHUGEPAGE, they could always issue multiple MADV_COLLAPSE
> > > operations around the excluded regions.
> > >
> > > In terms of use cases, I don't have a concrete example, but a user
> > > could hypothetically choose to exclude regions from management from
> > > khugepaged, but still be able to collapse the memory themselves,
> > > when/if they deem appropriate.
> >
> > I see. It seems you thought MADV_COLLAPSE actually unsets
> > VM_NOHUGEPAGE, and is kind of equal to MADV_HUGEPAGE + doing collapse
> > right away, right? To some degree, it makes some sense.
>
> Currently, MADV_COLLAPSE doesn't alter the vma flags at all - it just
> ignores VM_NOHUGEPAGE, and so it's not really the same as
> MADV_HUGEPAGE + MADV_COLLAPSE (which would set VM_HUGEPAGE in addition
> to clearing VM_NOHUGEPAGE). If my use case has any merit (and I'm not
> sure it does) then we don't want to be altering the vma flags since we
> don't want to touch khugepaged behavior.
>
> > If this is the
> > behavior you'd like to achieve, I'd suggest making it more explicit,
> > for example, setting VM_HUGEPAGE for the MADV_COLLAPSE area rather
> > than ignore or change vm flags silently. When using madvise mode, but
> > not having VM_HUGEPAGE set, the vma check should fail in the current
> > code (I didn't look hard if you already covered this or not).
> >
>
> You're correct, this will fail, since it's following the same
> semantics as the fault path. I see what you're saying though; that
> perhaps this is inconsistent with my above reasoning that "the user
> asked to collapse this memory, and so we should do it". If so, then
> perhaps MADV_COLLAPSE just ignores madise mode and VM_[NO]HUGEPAGE
> entirely for the purposes of eligibility, and only uses it for the
> purposes of determining gfp flags for compaction/reclaim. Pushing that
> further, compaction/reclaim could entirely be specified by the user
> using a process_madvise(2) flag (later in the series, we do something
> like this).

Anyway I think we could have two options for MADV_COLLAPSE:

1. Just treat it as a hint (nice to have, best effort). It should obey
all the settings. Skip VM_NOHUGEPAGE vmas or vmas without VM_HUGEPAGE
if madvise mode, etc.

2. Much stronger. It equals MADV_HUGEPAGE + synchronous collapse. It
should set vma flags properly as I suggested.

Either is fine to me. But I don't prefer something in between personally.

>
>
> > >
> > > > >
> > > > > Add a vm_flags_ignore argument to hugepage_vma_revalidate_pmd_count()
> > > > > which can be used to ignore vm flags used when considering thp
> > > > > eligibility.
> > > > >
> > > > > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > > > > ---
> > > > >  mm/khugepaged.c | 18 ++++++++++++------
> > > > >  1 file changed, 12 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > > index 1d20be47bcea..ecbd3fc41c80 100644
> > > > > --- a/mm/khugepaged.c
> > > > > +++ b/mm/khugepaged.c
> > > > > @@ -964,10 +964,14 @@ khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
> > > > >  #endif
> > > > >
> > > > >  /*
> > > > > - * Revalidate a vma's eligibility to collapse nr hugepages.
> > > > > + * Revalidate a vma's eligibility to collapse nr hugepages. vm_flags_ignore
> > > > > + * can be used to ignore certain vma_flags that would otherwise be checked -
> > > > > + * the principal example being VM_NOHUGEPAGE which is ignored in madvise
> > > > > + * collapse context.
> > > > >   */
> > > > >  static int hugepage_vma_revalidate_pmd_count(struct mm_struct *mm,
> > > > >                                              unsigned long address, int nr,
> > > > > +                                            unsigned long vm_flags_ignore,
> > > > >                                              struct vm_area_struct **vmap)
> > > > >  {
> > > > >         struct vm_area_struct *vma;
> > > > > @@ -986,7 +990,7 @@ static int hugepage_vma_revalidate_pmd_count(struct mm_struct *mm,
> > > > >         hend = vma->vm_end & HPAGE_PMD_MASK;
> > > > >         if (address < hstart || (address + nr * HPAGE_PMD_SIZE) > hend)
> > > > >                 return SCAN_ADDRESS_RANGE;
> > > > > -       if (!hugepage_vma_check(vma, vma->vm_flags))
> > > > > +       if (!hugepage_vma_check(vma, vma->vm_flags & ~vm_flags_ignore))
> > > > >                 return SCAN_VMA_CHECK;
> > > > >         /* Anon VMA expected */
> > > > >         if (!vma->anon_vma || vma->vm_ops)
> > > > > @@ -1000,9 +1004,11 @@ static int hugepage_vma_revalidate_pmd_count(struct mm_struct *mm,
> > > > >   */
> > > > >
> > > > >  static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> > > > > +                                  unsigned long vm_flags_ignore,
> > > > >                                    struct vm_area_struct **vmap)
> > > > >  {
> > > > > -       return hugepage_vma_revalidate_pmd_count(mm, address, 1, vmap);
> > > > > +       return hugepage_vma_revalidate_pmd_count(mm, address, 1,
> > > > > +                       vm_flags_ignore, vmap);
> > > > >  }
> > > > >
> > > > >  /*
> > > > > @@ -1043,7 +1049,7 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> > > > >                 /* do_swap_page returns VM_FAULT_RETRY with released mmap_lock */
> > > > >                 if (ret & VM_FAULT_RETRY) {
> > > > >                         mmap_read_lock(mm);
> > > > > -                       if (hugepage_vma_revalidate(mm, haddr, &vma)) {
> > > > > +                       if (hugepage_vma_revalidate(mm, haddr, VM_NONE, &vma)) {
> > > > >                                 /* vma is no longer available, don't continue to swapin */
> > > > >                                 trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> > > > >                                 return false;
> > > > > @@ -1200,7 +1206,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> > > > >         count_memcg_page_event(new_page, THP_COLLAPSE_ALLOC);
> > > > >
> > > > >         mmap_read_lock(mm);
> > > > > -       result = hugepage_vma_revalidate(mm, address, &vma);
> > > > > +       result = hugepage_vma_revalidate(mm, address, VM_NONE, &vma);
> > > > >         if (result) {
> > > > >                 mmap_read_unlock(mm);
> > > > >                 goto out_nolock;
> > > > > @@ -1232,7 +1238,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> > > > >          */
> > > > >         mmap_write_lock(mm);
> > > > >
> > > > > -       result = hugepage_vma_revalidate(mm, address, &vma);
> > > > > +       result = hugepage_vma_revalidate(mm, address, VM_NONE, &vma);
> > > > >         if (result)
> > > > >                 goto out_up_write;
> > > > >         /* check if the pmd is still valid */
> > > > > --
> > > > > 2.35.1.616.g0bdcbb4464-goog
> > > > >