Re: [PATCH v3 08/12] mm/collapse: separate scanning a PTE table from collapsing it

[Baolin Wang]

On 9/16/26 5:31 PM, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
>
> A collapse is two jobs.  One reads a PTE table under mmap_lock and decides
> whether the range is worth collapsing.  The other allocates, isolates,
> copies and flushes, and wants the lock given up first.
>
> collapse_single_pmd() did both, so the boundary between them was somewhere
> in the middle of a function.
>
> Give each half its own function:
>
>    - collapse_scan_pmd() scans one table and only reads.  The anonymous
>      scan that used to carry that name keeps its body as
>      collapse_scan_anon_pmd(), and collapse_scan_pmd() is now the entry
>      that picks the anonymous or the file side.
>
>    - collapse_run_pmd() does the collapse the scan asked for, and is
>      handed what the scan returned.  SCAN_SUCCEED means there is
>      something to collapse.  SCAN_PTE_MAPPED_HUGEPAGE means the page
>      cache already holds the PMD folio and only the PTE table is left to
>      retract.  Both are work for the run; anything else is why there is
>      nothing to do.
>
> collapse_single_pmd() is now the two of them with the mmap_lock drop in
> between, so its callers see what they saw before.
>
> What the scan found and the run needs travels in collapse_control.  For
> an anonymous table that is the orders and the referenced and swapped-out
> counts, which mthp_collapse() and collapse_huge_page() now read from
> there instead of taking as arguments.  For a file it is the file itself
> and the offset in it.
>
> The file side moves with the anonymous one.  collapse_scan_file() used to
> run with mmap_lock already given up, and called collapse_file() itself
> when the page cache looked worth it.  It now runs under the lock like the
> anonymous scan and only reads; the run does the collapse.  A file
> collapse works on the page cache and never sees a VMA, so the scan takes
> the file reference while it still has one and the run gives it back.
>
> That changes what a refused file table costs khugepaged.  Every file
> table it scanned used to end its pass over that mm, because the lock had
> been dropped to scan it; now only a table it goes on to collapse does.
>
> Two things on the file side stop being rescanned.  When the page cache
> already holds the PMD folio, the scan says so and the run goes straight
> to retracting the PTE table.  A run that refuses dirty pages and may
> write them back retries collapse_file() alone.  The checks the scan makes
> ahead of it are ones collapse_file() repeats under the page cache lock.
>
> Tracing changes with it.  mm_khugepaged_scan_pmd and
> mm_khugepaged_scan_file used to fire after the collapse, so for an
> accepted table their status field carried what the collapse made of it.
> They now fire before it and read SCAN_SUCCEED for an accepted table.  What
> the collapse then made of it is for mm_collapse_huge_page and
> mm_khugepaged_collapse_file to report.
>
> Assisted-by: LLM
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>