Message-ID: <20231204142146.91437-1-david@redhat.com>
Date:   Mon,  4 Dec 2023 15:21:07 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Ryan Roberts <ryan.roberts@....com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Peter Xu <peterx@...hat.com>
Subject: [PATCH RFC 00/39] mm/rmap: interface overhaul

Baed on mm-stable from a couple of days.

This series proposes an overhaul to our rmap interface, to get rid of the
"bool compound" / RMAP_COMPOUND parameter with the goal of making the
interface less error prone, more future proof, and more natural to extend
to "batching". Also, this converts the interface to always consume
folio+subpage, which speeds up operations on large folios.

Further, this series adds PTE-batching variants for 4 rmap functions,
whereby only folio_add_anon_rmap_ptes() is used for batching in this series
when PTE-remapping a PMD-mapped THP.

Ryan has series where we would make use of folio_remove_rmap_ptes() [1]
-- he carries his own batching variant right now -- and
folio_try_dup_anon_rmap_ptes()/folio_dup_file_rmap_ptes() [2].

There is some overlap with both series (and some other work, like
multi-size THP [3]), so that will need some coordination, and likely a
stepwise inclusion.

I got that started [4], but it made sense to show the whole picture. The
patches of [4] are contained in here, with one additional patch added
("mm/rmap: introduce and use hugetlb_try_share_anon_rmap()") and some
slight patch description changes.

In general, RMAP batching is an important optimization for PTE-mapped
THP, especially once we want to move towards a total mapcount or further,
as shown with my WIP patches on "mapped shared vs. mapped exclusively" [5].

The rmap batching part of [5] is also contained here in a slightly reworked
fork [and I found a bug du to the "compound" parameter handling in these
patches that should be fixed here :) ].

This series performs a lot of folio conversion, that could be separated
if there is a good reason. Most of the added LOC in the diff are only due
to documentation.

As we're moving to a pte/pmd interface where we clearly express the
mapping granularity we are dealing with, we first get the remainder of
hugetlb out of the way, as it is special and expected to remain special: it
treats everything as a "single logical PTE" and only currently allows
entire mappings.

Even if we'd ever support partial mappings, I strongly
assume the interface and implementation will still differ heavily:
hopefull we can avoid working on subpages/subpage mapcounts completely and
only add a "count" parameter for them to enable batching.


New (extended) hugetlb interface that operate on entire folio:
 * hugetlb_add_new_anon_rmap() -> Already existed
 * hugetlb_add_anon_rmap() -> Already existed
 * hugetlb_try_dup_anon_rmap()
 * hugetlb_try_share_anon_rmap()
 * hugetlb_add_file_rmap()
 * hugetlb_remove_rmap()

New "ordinary" interface for small folios / THP::
 * folio_add_new_anon_rmap() -> Already existed
 * folio_add_anon_rmap_[pte|ptes|pmd]()
 * folio_try_dup_anon_rmap_[pte|ptes|pmd]()
 * folio_try_share_anon_rmap_[pte|pmd]()
 * folio_add_file_rmap_[pte|ptes|pmd]()
 * folio_dup_file_rmap_[pte|ptes|pmd]()
 * folio_remove_rmap_[pte|ptes|pmd]()

folio_add_new_anon_rmap() will always map at the biggest granularity
possible (currently, a single PMD to cover a PMD-sized THP). Could be
extended if ever required.

In the future, we might want "_pud" variants and eventually "_pmds" variants
for batching. Further, if hugepd is ever a thing outside hugetlb code,
we might want some variants for that. All stuff for the distant future.


I ran some simple microbenchmarks from [5] on an Intel(R) Xeon(R) Silver
4210R: munmap(), fork(), cow, MADV_DONTNEED on each PTE ... and PTE
remapping PMD-mapped THPs on 1 GiB of memory.

For small folios, there is barely a change (< 1 % performance improvement),
whereby fork() still stands out with 0.74% performance improvement, but
it might be just some noise. Folio optimizations don't help that much
with small folios.

For PTE-mapped THP:
* PTE-remapping a PMD-mapped THP is more than 10% faster.
  -> RMAP batching
* fork() is more than 4% faster.
  -> folio conversion
* MADV_DONTNEED is 2% faster
  -> folio conversion
* COW by writing only a single byte on a COW-shared PTE
  -> folio conversion
* munmap() is only slightly faster (< 1%).

[1] https://lkml.kernel.org/r/20230810103332.3062143-1-ryan.roberts@arm.com
[2] https://lkml.kernel.org/r/20231204105440.61448-1-ryan.roberts@arm.com
[3] https://lkml.kernel.org/r/20231204102027.57185-1-ryan.roberts@arm.com
[4] https://lkml.kernel.org/r/20231128145205.215026-1-david@redhat.com
[5] https://lkml.kernel.org/r/20231124132626.235350-1-david@redhat.com

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Ryan Roberts <ryan.roberts@....com>
Cc: Yin Fengwei <fengwei.yin@...el.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Muchun Song <muchun.song@...ux.dev>
Cc: Peter Xu <peterx@...hat.com>

David Hildenbrand (39):
  mm/rmap: rename hugepage_add* to hugetlb_add*
  mm/rmap: introduce and use hugetlb_remove_rmap()
  mm/rmap: introduce and use hugetlb_add_file_rmap()
  mm/rmap: introduce and use hugetlb_try_dup_anon_rmap()
  mm/rmap: introduce and use hugetlb_try_share_anon_rmap()
  mm/rmap: add hugetlb sanity checks
  mm/rmap: convert folio_add_file_rmap_range() into
    folio_add_file_rmap_[pte|ptes|pmd]()
  mm/memory: page_add_file_rmap() -> folio_add_file_rmap_[pte|pmd]()
  mm/huge_memory: page_add_file_rmap() -> folio_add_file_rmap_pmd()
  mm/migrate: page_add_file_rmap() -> folio_add_file_rmap_pte()
  mm/userfaultfd: page_add_file_rmap() -> folio_add_file_rmap_pte()
  mm/rmap: remove page_add_file_rmap()
  mm/rmap: factor out adding folio mappings into __folio_add_rmap()
  mm/rmap: introduce folio_add_anon_rmap_[pte|ptes|pmd]()
  mm/huge_memory: batch rmap operations in __split_huge_pmd_locked()
  mm/huge_memory: page_add_anon_rmap() -> folio_add_anon_rmap_pmd()
  mm/migrate: page_add_anon_rmap() -> folio_add_anon_rmap_pte()
  mm/ksm: page_add_anon_rmap() -> folio_add_anon_rmap_pte()
  mm/swapfile: page_add_anon_rmap() -> folio_add_anon_rmap_pte()
  mm/memory: page_add_anon_rmap() -> folio_add_anon_rmap_pte()
  mm/rmap: remove page_add_anon_rmap()
  mm/rmap: remove RMAP_COMPOUND
  mm/rmap: introduce folio_remove_rmap_[pte|ptes|pmd]()
  kernel/events/uprobes: page_remove_rmap() -> folio_remove_rmap_pte()
  mm/huge_memory: page_remove_rmap() -> folio_remove_rmap_pmd()
  mm/khugepaged: page_remove_rmap() -> folio_remove_rmap_pte()
  mm/ksm: page_remove_rmap() -> folio_remove_rmap_pte()
  mm/memory: page_remove_rmap() -> folio_remove_rmap_pte()
  mm/migrate_device: page_remove_rmap() -> folio_remove_rmap_pte()
  mm/rmap: page_remove_rmap() -> folio_remove_rmap_pte()
  Documentation: stop referring to page_remove_rmap()
  mm/rmap: remove page_remove_rmap()
  mm/rmap: convert page_dup_file_rmap() to
    folio_dup_file_rmap_[pte|ptes|pmd]()
  mm/rmap: introduce folio_try_dup_anon_rmap_[pte|ptes|pmd]()
  mm/huge_memory: page_try_dup_anon_rmap() ->
    folio_try_dup_anon_rmap_pmd()
  mm/memory: page_try_dup_anon_rmap() -> folio_try_dup_anon_rmap_pte()
  mm/rmap: remove page_try_dup_anon_rmap()
  mm: convert page_try_share_anon_rmap() to
    folio_try_share_anon_rmap_[pte|pmd]()
  mm/rmap: rename COMPOUND_MAPPED to ENTIRELY_MAPPED

 Documentation/mm/transhuge.rst       |   4 +-
 Documentation/mm/unevictable-lru.rst |   4 +-
 include/linux/mm.h                   |   6 +-
 include/linux/rmap.h                 | 380 +++++++++++++++++++-----
 kernel/events/uprobes.c              |   2 +-
 mm/gup.c                             |   2 +-
 mm/huge_memory.c                     |  85 +++---
 mm/hugetlb.c                         |  21 +-
 mm/internal.h                        |  12 +-
 mm/khugepaged.c                      |  17 +-
 mm/ksm.c                             |  15 +-
 mm/memory-failure.c                  |   4 +-
 mm/memory.c                          |  60 ++--
 mm/migrate.c                         |  12 +-
 mm/migrate_device.c                  |  41 +--
 mm/mmu_gather.c                      |   2 +-
 mm/rmap.c                            | 422 ++++++++++++++++-----------
 mm/swapfile.c                        |   2 +-
 mm/userfaultfd.c                     |   2 +-
 19 files changed, 709 insertions(+), 384 deletions(-)

-- 
2.41.0

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