On Wed, Mar 9, 2022 at 4:41 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Wed, Mar 9, 2022 at 4:01 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
> >
> > > On Tue, Mar 8, 2022 at 1:35 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> 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).
> >
> > > >
> > > > 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@xxxxxxxxxx>
> > > > ---
> > > > 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
> > > >
