[PATCH v5 0/5] variable-order, large folios for anonymous memory

Wed Aug 16 01:11:09 PDT 2023

> On Aug 10, 2023, at 23:29, Ryan Roberts <ryan.roberts at arm.com> wrote:
> 
> Hi All,
> 
> This is v5 of a series to implement variable order, large folios for anonymous
> memory. (currently called "LARGE_ANON_FOLIO", previously called "FLEXIBLE_THP").
> The objective of this is to improve performance by allocating larger chunks of
> memory during anonymous page faults:
> 
> 1) Since SW (the kernel) is dealing with larger chunks of memory than base
>   pages, there are efficiency savings to be had; fewer page faults, batched PTE
>   and RMAP manipulation, reduced lru list, etc. In short, we reduce kernel
>   overhead. This should benefit all architectures.
> 2) Since we are now mapping physically contiguous chunks of memory, we can take
>   advantage of HW TLB compression techniques. A reduction in TLB pressure
>   speeds up kernel and user space. arm64 systems have 2 mechanisms to coalesce
>   TLB entries; "the contiguous bit" (architectural) and HPA (uarch).
> 
> This patch set deals with the SW side of things (1). (2) is being tackled in a
> separate series. The new behaviour is hidden behind a new Kconfig switch,
> LARGE_ANON_FOLIO, which is disabled by default. Although the eventual aim is to
> enable it by default.
> 
> My hope is that we are pretty much there with the changes at this point;
> hopefully this is sufficient to get an initial version merged so that we can
> scale up characterization efforts. Although they should not be merged until the
> prerequisites are complete. These are in progress and tracked at [5].
> 
> This series is based on mm-unstable (ad3232df3e41).
> 
> I'm going to be out on holiday from the end of today, returning on 29th
> August. So responses will likely be patchy, as I'm terrified of posting
> to list from my phone!
> 
> 
> Testing
> -------
> 
> This version adds patches to mm selftests so that the cow tests explicitly test
> large anon folios, in the same way that thp is tested. When enabled you should
> see something similar at the start of the test suite:
> 
>  # [INFO] detected large anon folio size: 32 KiB
> 
> Then the following results are expected. The fails and skips are due to existing
> issues in mm-unstable:
> 
>  # Totals: pass:207 fail:16 xfail:0 xpass:0 skip:85 error:0
> 
> Existing mm selftests reveal 1 regression in khugepaged tests when
> LARGE_ANON_FOLIO is enabled:
> 
>  Run test: collapse_max_ptes_none (khugepaged:anon)
>  Maybe collapse with max_ptes_none exceeded.... Fail
>  Unexpected huge page
> 
> I believe this is because khugepaged currently skips non-order-0 pages when
> looking for collapse opportunities and should get fixed with the help of
> DavidH's work to create a mechanism to precisely determine shared vs exclusive
> pages.
> 
> 
> Changes since v4 [4]
> --------------------
> 
>  - Removed "arm64: mm: Override arch_wants_pte_order()" patch; arm64
>    now uses the default order-3 size. I have moved this patch over to
>    the contpte series.
>  - Added "mm: Allow deferred splitting of arbitrary large anon folios" back
>    into series. I originally removed this at v2 to add to a separate series,
>    but that series has transformed significantly and it no longer fits, so
>    bringing it back here.
>  - Reintroduced dependency on set_ptes(); Originally dropped this at v2, but
>    set_ptes() is in mm-unstable now.
>  - Updated policy for when to allocate LAF; only fallback to order-0 if
>    MADV_NOHUGEPAGE is present or if THP disabled via prctl; no longer rely on
>    sysfs's never/madvise/always knob.
>  - Fallback to order-0 whenever uffd is armed for the vma, not just when
>    uffd-wp is set on the pte.
>  - alloc_anon_folio() now returns `strucxt folio *`, where errors are encoded
>    with ERR_PTR().
> 
>  The last 3 changes were proposed by Yu Zhao - thanks!
> 
> 
> Changes since v3 [3]
> --------------------
> 
>  - Renamed feature from FLEXIBLE_THP to LARGE_ANON_FOLIO.
>  - Removed `flexthp_unhinted_max` boot parameter. Discussion concluded that a
>    sysctl is preferable but we will wait until real workload needs it.
>  - Fixed uninitialized `addr` on read fault path in do_anonymous_page().
>  - Added mm selftests for large anon folios in cow test suite.
> 
> 
> Changes since v2 [2]
> --------------------
> 
>  - Dropped commit "Allow deferred splitting of arbitrary large anon folios"
>      - Huang, Ying suggested the "batch zap" work (which I dropped from this
>        series after v1) is a prerequisite for merging FLXEIBLE_THP, so I've
>        moved the deferred split patch to a separate series along with the batch
>        zap changes. I plan to submit this series early next week.
>  - Changed folio order fallback policy
>      - We no longer iterate from preferred to 0 looking for acceptable policy
>      - Instead we iterate through preferred, PAGE_ALLOC_COSTLY_ORDER and 0 only
>  - Removed vma parameter from arch_wants_pte_order()
>  - Added command line parameter `flexthp_unhinted_max`
>      - clamps preferred order when vma hasn't explicitly opted-in to THP
>  - Never allocate large folio for MADV_NOHUGEPAGE vma (or when THP is disabled
>    for process or system).
>  - Simplified implementation and integration with do_anonymous_page()
>  - Removed dependency on set_ptes()
> 
> 
> Changes since v1 [1]
> --------------------
> 
>  - removed changes to arch-dependent vma_alloc_zeroed_movable_folio()
>  - replaced with arch-independent alloc_anon_folio()
>      - follows THP allocation approach
>  - no longer retry with intermediate orders if allocation fails
>      - fallback directly to order-0
>  - remove folio_add_new_anon_rmap_range() patch
>      - instead add its new functionality to folio_add_new_anon_rmap()
>  - remove batch-zap pte mappings optimization patch
>      - remove enabler folio_remove_rmap_range() patch too
>      - These offer real perf improvement so will submit separately
>  - simplify Kconfig
>      - single FLEXIBLE_THP option, which is independent of arch
>      - depends on TRANSPARENT_HUGEPAGE
>      - when enabled default to max anon folio size of 64K unless arch
>        explicitly overrides
>  - simplify changes to do_anonymous_page():
>      - no more retry loop
> 
> 
> [1] https://lore.kernel.org/linux-mm/20230626171430.3167004-1-ryan.roberts@arm.com/
> [2] https://lore.kernel.org/linux-mm/20230703135330.1865927-1-ryan.roberts@arm.com/
> [3] https://lore.kernel.org/linux-mm/20230714160407.4142030-1-ryan.roberts@arm.com/
> [4] https://lore.kernel.org/linux-mm/20230726095146.2826796-1-ryan.roberts@arm.com/
> [5] https://lore.kernel.org/linux-mm/f8d47176-03a8-99bf-a813-b5942830fd73@arm.com/
> 
> 
> Thanks,
> Ryan
> 
> Ryan Roberts (5):
>  mm: Allow deferred splitting of arbitrary large anon folios
>  mm: Non-pmd-mappable, large folios for folio_add_new_anon_rmap()
>  mm: LARGE_ANON_FOLIO for improved performance
>  selftests/mm/cow: Generalize do_run_with_thp() helper
>  selftests/mm/cow: Add large anon folio tests
> 
> include/linux/pgtable.h          |  13 ++
> mm/Kconfig                       |  10 ++
> mm/memory.c                      | 144 +++++++++++++++++--
> mm/rmap.c                        |  31 +++--
> tools/testing/selftests/mm/cow.c | 229 ++++++++++++++++++++++---------
> 5 files changed, 347 insertions(+), 80 deletions(-)
> 
> --
> 2.25.1
> 

I know Ryan is away currently, but as I can’t find the base commit mentioned in the cover letter to be based off of can anybody point me to it so I can use b4 for applying the series and test?

Thanks,
Itaru.