Re: [PATCH v4 0/3] Optimize large folio interaction with deferred split

[Ryan Roberts <ryan.roberts@arm.com>]

Hi Andrew,

After discussion about this in Matthew's THP Cabal, we have decided to take a
different approach with this patch set. Could you therefore remove it from
mm-unstable, please? Sorry about the noise.

I'm going to try 2 different approaches:
 - avoid the split lock contention by using mmu gather (suggested by Kirill)
 - expand the zap pte batching to also cover file folios (as requested by Yu).

Thanks,
Ryan



On 27/07/2023 15:18, Ryan Roberts wrote:
> Hi All,
> 
> This is v4 of a small series in support of my work to enable the use of large
> folios for anonymous memory (known as "FLEXIBLE_THP" or "LARGE_ANON_FOLIO") [5].
> It first makes it possible to add large, non-pmd-mappable folios to the deferred
> split queue. Then it modifies zap_pte_range() to batch-remove spans of
> physically contiguous pages from the rmap, which means that in the common case,
> we elide the need to ever put the folio on the deferred split queue, thus
> reducing lock contention and improving performance.
> 
> This becomes more visible once we have lots of large anonymous folios in the
> system, and Huang Ying has suggested solving this needs to be a prerequisit for
> merging the main FLEXIBLE_THP/LARGE_ANON_FOLIO work.
> 
> The series applies on top of v6.5-rc3 and a branch is available at [4].
> 
> NOTE: v3 is currently in mm-unstable and has a bug that affects s390, which this
> version fixes.
> 
> 
> Changes since v3 [3]
> --------------------
> 
> - Fixed bug reported on s390 [6]
>     - Since s390 enables MMU_GATHER_NO_GATHER, __tlb_remove_page() causes a ref
>       to be dropped on the page, but we were still using the page after that
>       function call.
>     - Fix by using folio_get()/folio_put() to guarrantee lifetime of page
>     - Thanks to Nathan Chancellor for the bug report and helping me get set up
>       with s390!
> - Don't use batch path if folio is not large
> 
> 
> Changes since v2 [2]
> --------------------
> 
> - patch 2: Reworked at Yu Zhou's request to reduce duplicated code.
>     - page_remove_rmap() now forwards to folio_remove_rmap_range() for the
>       !compound (PMD mapped) case.
>     - Both page_remove_rmap() and folio_remove_rmap_range() share common
>       epilogue via new helper function __remove_rmap_finish().
>     - As a result of the changes, I've removed the previous Reviewed-bys.
> - other 2 patches are unchanged.
> 
> 
> Changes since v1 [1]
> --------------------
> 
> - patch 2: Modified doc comment for folio_remove_rmap_range()
> - patch 2: Hoisted _nr_pages_mapped manipulation out of page loop so its now
>   modified once per folio_remove_rmap_range() call.
> - patch 2: Added check that page range is fully contained by folio in
>   folio_remove_rmap_range()
> - patch 2: Fixed some nits raised by Huang, Ying for folio_remove_rmap_range()
> - patch 3: Support batch-zap of all anon pages, not just those in anon vmas
> - patch 3: Renamed various functions to make their use clear
> - patch 3: Various minor refactoring/cleanups
> - Added Reviewed-By tags - thanks!
> 
> 
> [1] https://lore.kernel.org/linux-mm/20230717143110.260162-1-ryan.roberts@arm.com/
> [2] https://lore.kernel.org/linux-mm/20230719135450.545227-1-ryan.roberts@arm.com/
> [3] https://lore.kernel.org/linux-mm/20230720112955.643283-1-ryan.roberts@arm.com/
> [4] https://gitlab.arm.com/linux-arm/linux-rr/-/tree/features/granule_perf/deferredsplit-lkml_v4
> [5] https://lore.kernel.org/linux-mm/20230714160407.4142030-1-ryan.roberts@arm.com/
> [6] https://lore.kernel.org/linux-mm/20230726161942.GA1123863@dev-arch.thelio-3990X/
> 
> Thanks,
> Ryan
> 
> 
> Ryan Roberts (3):
>   mm: Allow deferred splitting of arbitrary large anon folios
>   mm: Implement folio_remove_rmap_range()
>   mm: Batch-zap large anonymous folio PTE mappings
> 
>  include/linux/rmap.h |   2 +
>  mm/memory.c          | 132 +++++++++++++++++++++++++++++++++++++++++++
>  mm/rmap.c            | 125 ++++++++++++++++++++++++++++++----------
>  3 files changed, 229 insertions(+), 30 deletions(-)
> 
> --
> 2.25.1
>