linux-kernel - Re: [PATCH v4 0/6] __folio

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Message-ID: <9696c72d-8b28-4658-adab-11c8eb61c2ce@lucifer.local>
Date: Fri, 18 Jul 2025 06:26:30 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Carpenter <dan.carpenter@...aro.org>,
        Antonio Quartulli <antonio@...delbit.com>,
        Hugh Dickins <hughd@...gle.com>,
        Kirill Shutemov <k.shutemov@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Balbir Singh <balbirs@...dia.com>,
        Matthew Brost <matthew.brost@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/6] __folio_split() clean up

On Thu, Jul 17, 2025 at 10:29:54PM -0400, Zi Yan wrote:
> Hi Andrew,
>
> This series replaces both [PATCH v3 0/2] __folio_split() clean up
> and [PATCH] mm/huge_memory: refactor after-split (page) cache code.
>
> Hi Lorenzo,
>
> I addressed all of your comments except renaming folio to origin_folio,
> since I find that might either cause confusion or require a lot of code
> churn. folio variable points to the original folio throughout
> __folio_split() and using origin_folio in the middle of __folio_split()
> is confusing as one might wonder if origin_folio is different from or
> the same as folio. The alternative is to rename all folio to origin_folio
> in __folio_split(). That seems to be unnecessary code churn.

Sounds reasonable! Cheers :)

>
> Hi all,
>
> This patchset refactors __folio_split() and __split_unmapped_folio() to:
> 1. make __split_unmapped_folio() reusable for splitting unmapped
>    folios. It avoids the need for a new boolean unmapped parameter to guard
>    mapping-related code when __split_unmapped_folio() is reused to split
>    unmapped folios.
> 2. improve code readability and prevent smatch/coverity checkers from
>    complaining about NULL mapping referencing.
>
> An additional benefit for __split_unmapped_folio() refactoring is that
> __split_unmapped_folio() could be called on after-split folios by
> __folio_split(). It can enable new split methods. For example, at deferred
> split time, unmapped subpages can scatter arbitrarily within a large folio,
> neither uniform nor non-uniform split can maximize after-split folio orders
> for mapped subpages. The hope is that by calling __split_unmapped_folio()
> multiple times, a better split result can be achieved.
>
> The patchset is based on mm-new with aforementioned two patchsets
> reverted. It passes mm selftests.
>
> Changelog
> ===
> From V3[4]:
> 1. Split up Patch 1 into incremental changes:
>     a. Patch 1 moves code out of __split_unmapped_folio();
>     b. Patch 2 removes after_split label in __split_unmapped_folio();
>     c. Patch 3 refactors __folio_split() to deduplicate code;
>     d. Patch 4 converts VM_BUGs to VM_WARMs;
> 2. Added "mm/huge_memory: refactor after-split (page) cache code"
>    patch[5] to this series.
> 3. Added remap_flags to make remap_page() call easier to read.
> 4. Updated Patch 1 commit log to include variable rename information.
> 5. Converted additional VM_BUGs in __folio_split().
> 6. Renamed next_folio to end_folio to avoid confusion.
> 7. Added a comment about start for loop with folio_next(folio) instead
>    of just folio plus skipping folio in the loop body.
> 8. Dropped swapcache folio split check code from __split_unmapped_folio(),
>    since the check is already done at the beginning of __folio_split().
>
> From V2[3]:
> 1. Code format fixes
> 2. Restructured code to remove after_split goto label.
>
> From V1[2]:
> 1. Fixed indentations.
> 2. Used folio_expected_ref_count() to calculate ref_count instead of
>    open coding.
>
> [1] https://lore.kernel.org/linux-mm/94D8C1A4-780C-4BEC-A336-7D3613B54845@nvidia.com/
> [2] https://lore.kernel.org/linux-mm/20250711030259.3574392-1-ziy@nvidia.com/
> [3] https://lore.kernel.org/linux-mm/20250711182355.3592618-1-ziy@nvidia.com/
> [4] https://lore.kernel.org/linux-mm/20250714171823.3626213-1-ziy@nvidia.com/
> [5] https://lore.kernel.org/linux-mm/20250716171112.3666150-1-ziy@nvidia.com/
>
> Zi Yan (6):
>   mm/huge_memory: move unrelated code out of __split_unmapped_folio()
>   mm/huge_memory: remove after_split label in __split_unmapped_folio().
>   mm/huge_memory: deduplicate code in __folio_split().
>   mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split.
>   mm/huge_memory: get frozen folio refcount with
>     folio_expected_ref_count()
>   mm/huge_memory: refactor after-split (page) cache code.
>
>  mm/huge_memory.c | 317 ++++++++++++++++++++++++-----------------------
>  1 file changed, 165 insertions(+), 152 deletions(-)
>
> --
> 2.47.2
>