Re: [PATCH] mm/khugepaged: fix collapse_pte_mapped_thp() to allow anon_vma

[Yang Shi <shy828301@gmail.com>]

On Wed, Jan 4, 2023 at 1:20 AM David Hildenbrand <david@redhat.com> wrote:
>
> >> Or am I wrong?
> >>
> >>> Is anon_vma lock required?  Almost not: if any page other than expected
> >>> subpage of the non-anon huge page is found in the page table, collapse is
> >>> aborted without making any change.  However, it is possible that an anon
> >>> page was CoWed from this extent in another mm or vma, in which case a
> >>> concurrent lookup might look here: so keep it away while clearing pmd
> >>> (but perhaps we shall go back to using pmd_lock() there in future).
> >>>
> >>> Note that collapse_pte_mapped_thp() is exceptional in freeing a page table
> >>> without having cleared its ptes: I'm uneasy about that, and had thought
> >>> pte_clear()ing appropriate; but exclusive i_mmap lock does fix the problem,
> >>> and we would have to move the mmu_notification if clearing those ptes.
> >>>
> >>> Fixes: 8d3c106e19e8 ("mm/khugepaged: take the right locks for page table
> >>> retraction")
> >>> Signed-off-by: Hugh Dickins <hughd@google.com>
> >>> Cc: Jann Horn <jannh@google.com>
> >>> Cc: Yang Shi <shy828301@gmail.com>
> >>> Cc: David Hildenbrand <david@redhat.com>
> >>> Cc: Zach O'Keefe <zokeefe@google.com>
> >>> Cc: Song Liu <songliubraving@fb.com>
> >>> Cc: <stable@vger.kernel.org>    [5.4+]
> >>> ---
> >>> What this fixes is not a dangerous instability!  But I suggest Cc stable
> >>> because uprobes "healing" has regressed in that way, so this should follow
> >>> 8d3c106e19e8 into those stable releases where it was backported (and may
> >>> want adjustment there - I'll supply backports as needed).
> >>
> >> If it's really something that doesn't matter in practice (e.g., -1%
> >> performance while debugging :) ), I guess no CC is needed. If there are real
> >> production workloads that suffer, I guess ccing stable is fine.
> >
> > It's about recovering performance *after* debugging.  It is not something
> > that is of any value to me personally, nor (so far as I know) to anyone
> > whom I work with.  But it is something which Song Liu went to the trouble
> > to make possible in his "THP aware uprobe" series three years ago, and it
> > is something which Jann unintentionally regressed in his recent commit:
> > so I thought it proper to reinstate where regressed.
>
> Right, although I wonder if that original series fixed a real
> performance issue or was more a "this makes sense, let's just optimize
> this corner case by some serious complexity". I hope it's not the latter :)
>
> >
> > (What I do have more of an investment in, is for MADV_COLLAPSE to be able
> > to collapse some extents in a large vma where some other extent got CoWed,
> > so giving the whole vma an anon_vma.  But that's not an issue for -stable,
> > and I cannot tell you offhand whether undoing this anon_vma exclusion is
> > enough to enable that or not - I suspect not, I suspect a result code or
> > switch statement needs to be adjusted too.)
>
> Yeah, having a single COWed page in a large MAP_PRIVATE file/shmem
> mapping would disable collapse, so it's the right thing to do.
>
> Thinking about it some more, and the effective code change, stable
> doesn't sound wrong.
>
> >>
> >>
> >> Side note: set_huge_pmd() wins the award of "ugliest mm function of early
> >> 2023". I was briefly concerned how do_set_pmd() decides whether the PMD can be
> >> writable or not. Turns out it's communicated via vm_fault->flags. Just
> >> horrible.
> >
> > I firmly disagree - it's from 2022! and much too small to be ugliest;
> > but I haven't thought about the aspect that is bothering you there.
>
> The ugliest I stumbled over in early 2023 -- until January 2nd :D
>
> >
> > What's bothered me most about it, is the way its name, and the naming of
> > the do_set_pmd() it interfaces with, give no hint that they are entirely
> > about file (or shmem) vmas, and would not work right on anon vmas
> > (I forget whether it's just a matter of which stats updated, or more).
>
> Yes. I dug very deep into in-place collapse yesterday because I was
> briefly concerned about anon THP, and it took me longer to understand
> that whole machinery than it should (and that anon THP never ever
> collapse in-place).
>
> Some of that khugepaged stuff needs some *serious* cleanups and
> refactoring. do_set_pmd() is not an exception.
>
>
> Some more examples:
>
> if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
>         ...
>         hpage_collapse_scan_file()
> } else {
>         hpage_collapse_scan_pmd()
>         ...
> }
>
>
> 1) hpage_collapse_scan_pmd() is only for anon memory. Totally obvious
>     from the name. But why are we potentially calling it for VMAs that
>     are not applicable? For maximum David confusion?

IIRC the VMAs are checked before, what do you mean about "not
applicable"? But anyway khugepaged/MADV_COLLAPSE does release and
reacquire mmap_lock multiple times, so there are multiple places to
check VMAs validity.

>
> 2) "IS_ENABLED(CONFIG_SHMEM) && vma->vm_file" is also supposed to cover
>     ordinary file-thp. Totally obvious from the IS_ENABLED(CONFIG_SHMEM)
>     ... I probably spent 30minutes understanding what's happening here.
>     Just misleading and wrong without CONFIG_SHMEM.
>
>
> ... and what's easier to get than this magic set of boolean flags:
>
>         hugepage_vma_check(vma, vma->vm_flags, false, false, true)

This is not perfect. I was thinking about changing them to one flag,
just like TTU_ flags used by try_to_unmap(). That may make things
cleaner.

>
> ... and obviously
>         hugepage_vma_revalidate()
> is supposed to be a follow up to a previous
>         hugepage_vma_check()
> and totally different from
>         transhuge_vma_suitable()
>
> Hard to make it even less consistent.

This was after my cleanup, it was much messier before. And I did add
comments to make them more understandable, but anyway better naming is
definitely welcome.

>
> Also, it's very clear from the code that SCAN_PTE_MAPPED_HUGEPAGE only
> applies to file-thp, right? No.
>
> --
> Thanks,
>
> David / dhildenb
>