On Thu, Mar 10, 2022 at 10:17 AM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Thu, Mar 10, 2022 at 7:51 AM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
> >
> > On Wed, Mar 9, 2022 at 6:16 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
> > >
> > > On Wed, Mar 9, 2022 at 5:10 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
> > > >
> > > > 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).
> > >
> > > 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.
> > >
> >
> > Makes sense to be consistent. Of these, #1 seems the most
> > straightforward to use. Doing an MADV_COLLAPSE on a VM_NOHUGEPAGE vma
> > seems like a corner case. The more likely scenario is MADV_COLLAPSE on
> > an unflagged (neither VM_HUGEPAGE or VM_NOHUGEPAGE) vma - in which
> > case it's less intrusive to not additionally set VM_HUGEPAGE (though
> > the user can always do so if they wish). It's a little more consistent
> > with "always" mode, where MADV_HUGEPAGE isn't necessary for
> > eligibility. It'll also reduce some code complexity.
> >
> > I'll float one last option your way, however:
> >
> > 3. The collapsed region is always eligible, regardless of vma flags or
> > thp settings (except "never"?). For process_madvise(2), a flag will
> > explicitly specify defrag semantics.
>
> This is what I meant for #2 IIUC. Defrag could follow the system's
> defrag setting rather than the khugepaged's.
>
> But it may break s390 as David pointed out.
>
> >
> > This separates "async-hint" vs "sync-explicit" madvise requests.
> > MADV_[NO]HUGEPAGE are hints, and together with thp settings, advise
> > the kernel how to treat memory in the future. The kernel uses
> > VM_[NO]HUGEPAGE to aid with this. MADV_COLLAPSE, as an explicit
> > request, is free to define its own defrag semantics.
> >
> > This would allow flexibility to separately define async vs sync thp
> > policies. For example, highly tuned userspace applications that are
> > sensitive to unexpected latency might want to manage their hugepages
> > utilization themselves, and ask khugepaged to stay away. There is no
> > way in "always" mode to do this without setting VM_NOHUGEPAGE.
>
> I don't quite get why you set THP to always but don't want to
> khugepaged do its job. It may be slow, I think this is why you
> introduce MADV_COLLAPSE, right? But it doesn't mean khugepaged can't
> scan the same area, it just doesn't do any real work and waste some
> cpu cycles. But I guess MADV_COLLAPSE doesn't prevent the PMD/THP from
> being split, right? So khugepaged still plays a role to re-collapse
> the area without calling MADV_COLLAPSE over again and again.
>
Ya, I agree that the common case is that, if you are MADV_COLLAPSE'ing
memory, chances are you just want that memory backed by hugepages -
and so if that area were to be split, presumably we'd want khugepaged
to come and recollapse when possible.
I think the (possibly contrived) use case I was thinking about a
program that (a) didn't have ability to change thp settings
("always"), and (b) wanted to manage it's own hugepages. If a concrete
use case like this did arise, I think David H.'s suggestion of using
prctl(2) would work.
> >
> > > >
> > > >
> > > > > >
> > > > > > > >
> > > > > > > > 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
> > > > > > > >
