Re: [RFC 00/11] khugepaged: mTHP support

Nico Pache
2025-01-23T20:42:41Z

On Mon, Jan 20, 2025 at 5:49 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 16/01/2025 20:53, Nico Pache wrote:
> > On Thu, Jan 16, 2025 at 2:47 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>
> >> Hi Nico,
> > Hi Ryan!
> >>
> >> On 08/01/2025 23:31, Nico Pache wrote:
> >>> The following series provides khugepaged and madvise collapse with the
> >>> capability to collapse regions to mTHPs.
> >>
> >> It's great to see multiple solutions for this feature being posted; I guess that
> >> leaves us with the luxurious problem of figuring out an uber-patchset that
> >> incorporates the best of both? :)
> > I guess so! My motivation for developing this was inspired by my
> > 'defer' RFC. Which can't really live without khugepaged having mTHP
> > support (ie having 32k mTHP= always and global=defer doesnt make
> > sense).
>
> I'm not sure why that wouldn't make sense? setting global=defer would only be
> picked up for a given size that sets "inherit". So "32k=always, 2m=inherit,
> global=defer" is the same as "32k=always, 2m=defer"; which means you would try
> to allocate 32K directly in the fault handler and defer collapse to 2m to
> khugepaged. I guess where it would get difficult is if you set a size less than
> PMD-size to defer; at the moment khugepaged can't actually do that; it would
> just end up collapsing to 2M? Anyway, I'm rambling... I get your point.

Yeah looks like you found one of the issues. so defer means no pf time
(m)THPs. mthp sysctls need a "defer" entry, what does it mean to defer
globally and have a mthp size as always or inherit? I assume for
global=defer and mthps=always/inherit/defer, we defer at pf time and
can collapse the mthp. and gobal=always and mthp=defer, we always
allocate a thp, then khugepaged can scan for (m)thp collapse.

>
> >>
> >> I haven't had a chance to review your series in detail yet, but have a few
> >> questions below that will help me understand the key differences between your
> >> series and Dev's.
> >>
> >>>
> >>> To achieve this we generalize the khugepaged functions to no longer depend
> >>> on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages
> >>> (defined by MTHP_MIN_ORDER) that are fully utilized. This info is tracked
> >>> using a bitmap. After the PMD scan is done, we do binary recursion on the
> >>> bitmap to find the optimal mTHP sizes for the PMD range. The restriction
> >>> on max_ptes_none is removed during the scan, to make sure we account for
> >>> the whole PMD range. max_ptes_none is mapped to a 0-100 range to
> >>> determine how full a mTHP order needs to be before collapsing it.
> >>>
> >>> Some design choices to note:
> >>> - bitmap structures are allocated dynamically because on some arch's
> >>> (like PowerPC) the value of MTHP_BITMAP_SIZE cannot be computed at
> >>> compile time leading to warnings.
> >>
> >> We have MAX_PTRS_PER_PTE and friends though, which are worst case and compile
> >> time. Could these help avoid the dynamic allocation?
> >>
> >> MAX_PMD_ORDER = ilog2(MAX_PTRS_PER_PTE * PAGE_SIZE)
> > is the MAX_PMD_ORDER = PMD_ORDER? if not this might introduce weird
> > edge cases where PMD_ORDER < MAX_PMD_ORDER.
>
> No, MAX_PMD_ORDER becomes the largest order that could be configured at boot.
> PMD_ORDER is what is actually configured at boot. My understanding was that you
> were dynamically allocating your bitmap based on the runtime value of PMD_ORDER?
> I was just suggesting that you could allocate it statically (on stack or
> whatever) based on MAX_PMD_ORDER, for the worst-case requirement and only
> actually use the portion required by the runtime PMD_ORDER value. It avoids the
> kmalloc call.

I originally had this on the stack, but the PMD_ORDER gave me trouble
for ppc. Ill try this approach to get it back on the stack! Thanks!

>
> >
> >>
> >> Althogh to be honest, it's not super clear to me what the benefit of the bitmap
> >> is vs just iterating through the PTEs like Dev does; is there a significant cost
> >> saving in practice? On the face of it, it seems like it might be uneeded complexity.
> > The bitmap was to encode the state of PMD without needing rescanning
> > (or refactor a lot of code). We keep the scan runtime constant at 512
> > (for x86). Dev did some good analysis for this here
> > https://lore.kernel.org/lkml/23023f48-95c6-4a24-ac8b-aba4b1a441b4@arm.com/
> > This prevents needing to hold the read lock for longer, and prevents
> > needing to reacquire it too.
> >>>>> - The recursion is masked through a stack structure.
> >>> - A MTHP_MIN_ORDER was added to compress the bitmap, and ensure it was
> >>> 64bit on x86. This provides some optimization on the bitmap operations.
> >>> if other arches/configs that have larger than 512 PTEs per PMD want to
> >>> compress their bitmap further we can change this value per arch.
> >>
> >> So 1 bit in the bitmap represents 8 pages? And it will only be set if all 8
> >> pages are !pte_none()? I'm wondering what will happen if you have a pattern of 4
> >> set PTEs followed by 4 none PTEs, followed by 4 set PTEs... If 16K mTHP is
> >> enabled, you would want to collapse every other 16K block in this case, but I'm
> >> guessing with your scheme, all the bits will be clear and no collapse will
> >> occur? But for arm64 at least, collapsing to order-2 (16K) may be desired for HPA.
> >
> > Yeah on my V2 ive incorporated a threshold (like max_ptes_none) for
> > setting the bit. This will covert this case better (given a better
> > default max_ptes_none).
> > The way i see it 511 max_ptes_none is just wrong...
>
> You mean it's a bad default?
Yeah that's the better phrasing.
>
> > we should flip it
> > towards the lower end of the scale (ie 64), and the "always" THP
> > setting should ignore it (like madvise does).
>
> But user space can already get that behaviour by modifying the tunable, right?
> Isn't that just a user space policy choice?
technically yes, but shouldn't defaults reflect sane behavior? ofc
this is my opinion, some might think 511 is not a bad default at all.
My original perspective comes from the memory waste issue, 511 could
be really good for performance if you benefit from PMDs; hence why I
was also suggesting "always" ignores the max_ptes_none.
>
> One other thing that occurs to me regarding the bitmap; In the context of Dev's
> series, we have discussed policy for what to do when the source PTEs are backed
> by a large folio already. I'm guessing if you are making your
> smaller-than-PMD-size collapse decisions based solely on the bitmap, you won't
> be able to see when the PTEs are already collpsed for the target order? i.e.
> let's say you already have a 64K folio fully mapped in an aligned way. You
> wouldn't want to "re-collapse" it to 64K. Are you robust to this?

Yes, I am also skipping the order <= folio_order case.

>
> >
> >>
> >>>
> >>> Patch 1-2: Some refactoring to combine madvise_collapse and khugepaged
> >>> Patch 3: A minor "fix"/optimization
> >>> Patch 4: Refactor/rename hpage_collapse
> >>> Patch 5-7: Generalize khugepaged functions for arbitrary orders
> >>> Patch 8-11: The mTHP patches
> >>>
> >>> This series acts as an alternative to Dev Jain's approach [1]. The two
> >>> series differ in a few ways:
> >>> - My approach uses a bitmap to store the state of the linear scan_pmd to
> >>> then determine potential mTHP batches. Devs incorporates his directly
> >>> into the scan, and will try each available order.
> >>
> >> So if I'm understanding, the benefit of the bitmap is to remove the need to
> >> re-scan the "low" PTEs when moving to a lower order, which is what Dev's
> >> approach does? Are there not some locking/consistency issues to manage if not
> >> re-scanning?
> > Correct, so far i haven't found any issues (other than the bugs Dev
> > reported in his review)-- my fixed version of this RFC has been
> > running fine with no notable locking issues.
> >>
> >>> - Dev is attempting to optimize the locking, while my approach keeps the
> >>> locking changes to a minimum. I believe his changes are not safe for
> >>> uffd.
> >>
> >> I agree; let's keep the locking simple for the initial effort.
> >>
> >>> - Dev's changes only work for khugepaged not madvise_collapse (although
> >>> i think that was by choice and it could easily support madvise)
> >>
> >> I agree supporting MADV_COLLAPSE is good; what exactly are the semantics for it
> >> though? I think it ignores the sysfs settings (max_ptes_none and friends) so
> >> presumably it will continue to be much more greedy about collapsing to the
> >> highest possible order and only fall back to lower orders if the VMA boundaries
> >> force it to or if the higher order allocation fails?
> > Kind of, because I removed the max_ptes_none check during the scan,
> > and reintroduced it in the bitmap scan (without a madvise
> > restriction), MADV_COLLAPSE and khugepaged will work more similarly.
> >>
> >>> - Dev scales all khugepaged sysfs tunables by order, while im removing
> >>> the restriction of max_ptes_none and converting it to a scale to
> >>> determine a (m)THP threshold.
> >>
> >> I don't really understand this statement. You say you are removing the
> >> restriction of max_ptes_none. But then you say you scale it to determine a
> >> threshold. So are you honoring it or not? And if you're honouring it, how is
> >> your scaling method different to Dev's? What about the other tunables (shared
> >> and swap)?
> > I removed the max_ptes_none restriction during the initial scan, so we
> > can account for the full PMD (which is what happens with
> > max_ptes_none=511 anyways). Then max_ptes_none can be used with the
> > bitmap to calculate a threshold (max_ptes_none=64 == ~90% full) for
> > finding the optimal mTHP size.
> >
> > This RFC scales max_ptes_none to 0-100, but it has some really bad
> > rounding issues, so instead ive incorporated scaling (via bitshifting)
> > like Dev did in his series. Ive tested this and it's more accurate
> > now.
> >>
> >>> - Dev turns on khugepaged if any order is available while mine still
> >>> only runs if PMDs are enabled. I like Dev's approach and will most
> >>> likely do the same in my PATCH posting.
> >>
> >> Agreed. Also, we will want khugepaged to be able to scan VMAs (or parts of VMAs)
> >> that cover only a partial PMD entry. I think neither of your implementations
> >> currently do that. As I understand it, Dev's v2 will add that support. Is your
> >> approach ammeanable to this?
> >
> > Yes, I believe so. I'm working on adding this too.
> >
> >>
> >>> - mTHPs need their ref count updated to 1<<order, which Dev is missing.
> >>>
> >>> Patch 11 was inspired by one of Dev's changes.
> >>
> >> I think the 1 problem that emerged during review of Dev's series, which we don't
> >> have a proper solution to yet, is the issue of "creep", where regions can be
> >> collapsed to progressively higher orders through iterative scans. At each
> >> collapse, the required thresholds (e.g. max_ptes_none) are met, and the collapse
> >> effectively adds more non-none ptes so the next scan will then collapse to even
> >> higher order. Does your solution suffer from this (theoretical/edge case) issue?
> >> If not, how did you solve?
> >
> > Yes sadly it suffers from the same issue. bringing max_ptes_none much
> > lower as a default would "help".
> > I liked Zi Yan's solution of a per-VMA bit that gets set when
> > khugepaged collapses, and unset when the VMA changes (pf, realloc,
> > etc).
> > Then khugepaged can only operate on VMAs that dont have the bit set.
> > This way we only collapse once, unless the mapping was changed.
>
> Dev raised the issue in discussion against his series, that currently khugepaged
> doesn't scan the entire VMA, it scans to the first PMD that it collapses then
> moves to another VMA. I guess that's a fairness thing. So a VMA flag won't quite
> do the trick assuming we want to continue with that behavior. Perhaps we could
> keep a "cursor" in the VMA though, which describes the starting address of the
> next scan. We can move it forwards as we scan. And move it backwards when taking
> a fault. Still not perfect, but perhaps good enough?

I started playing around with some of these changes, it seems to work,
but David raised the issue that we can't expand vm_struct, so I need
to find a different solution.

>
> >
> > Could we map the new "non-none" pages to the zero page (rather than
> > actually zeroing the page), so they dont actually act as new "utilized
> > pages" and are still counted as none pages during the scan (until they
> > are written to)?
>
> I think you are propsing to use the zero page as a PTE marker to say "this
> region is scheduled for collapse"? In which case, why not just use a PTE
> marker... But you still have to do the collapse at some point (which I guess you
> are now deferring to the next page fault that hits one of those markers)? Once
> you have collapsed, you're still back to the original issue. So I don't think
> it's bought you anything except complexity and more latency :)

Ah ok i see! Thanks for clarifying
>
> Thanks,
> Ryan
>
> >
> >>
> >> Thanks,
> >> Ryan
> >
> > Cheers!
> > -- Nico
> >
> >>
> >>
> >>>
> >>> [1] https://lore.kernel.org/lkml/20241216165105.56185-1-dev.jain@arm.com/
> >>>
> >>> Nico Pache (11):
> >>> introduce khugepaged_collapse_single_pmd to collapse a single pmd
> >>> khugepaged: refactor madvise_collapse and khugepaged_scan_mm_slot
> >>> khugepaged: Don't allocate khugepaged mm_slot early
> >>> khugepaged: rename hpage_collapse_* to khugepaged_*
> >>> khugepaged: generalize hugepage_vma_revalidate for mTHP support
> >>> khugepaged: generalize alloc_charge_folio for mTHP support
> >>> khugepaged: generalize __collapse_huge_page_* for mTHP support
> >>> khugepaged: introduce khugepaged_scan_bitmap for mTHP support
> >>> khugepaged: add mTHP support
> >>> khugepaged: remove max_ptes_none restriction on the pmd scan
> >>> khugepaged: skip collapsing mTHP to smaller orders
> >>>
> >>> include/linux/khugepaged.h | 4 +-
> >>> mm/huge_memory.c | 3 +-
> >>> mm/khugepaged.c | 436 +++++++++++++++++++++++++------------
> >>> 3 files changed, 306 insertions(+), 137 deletions(-)
> >>>
> >>
> >
>