Re: [RFC PATCH v3 0/8] batch lookups in follow_page_mask()

[David Hildenbrand (Arm)]

On 8/11/26 04:51, Rik van Riel wrote:
> follow_page_mask() walks the page tables one page at a time, even when the
> caller asked for a whole run of contiguous pages. Every page of a large folio
> re-enters the pmd/pud/pte walk and re-takes the page table lock.
> 
> This series changes follow_page_mask() to return a page count and a fill an
> array of pages, instead of a single struct page, so a walker can hand back
> more than one page per call.
> 
> Patches 1 to 4 are preparation, no functional change:
> 
>   1: move __get_user_pages()'s open-coded pages[] fill and cache flush into a
>      gup_fill_pages() helper, which the rest of the series reuses.
>   2: convert the follow_page_mask()/follow_p4d_mask()/follow_pud_mask()/
>      follow_pmd_mask()/follow_page_pte() call chain to return a long instead of
>      a struct page pointer or ERR_PTR(). Every path still handles one page.
>   3: split the "commit to a resolved page" tail of follow_page_pte() into
>      follow_page_pte_commit().
>   4: split the "work out which page this PTE maps" half of follow_page_pte()
>      into follow_one_pte(), leaving one unlock and one exit.
> 
> Patch 5 has the huge page paths store the page and leave the array fill to
> follow_pud_mask()/follow_pmd_mask() after they unlock, so the cache flushes
> happen outside the pud/pmd critical section.
> 
> Patch 6 has follow_huge_pud()/follow_huge_pmd() report the huge page's real
> subpage count instead of a separate *page_mask output, and retires *page_mask
> and __get_user_pages()'s try_grab_folio() call and subpage loop.
> 
> Patch 7 walks every PTE in a page table in one follow_page_pte() call instead
> of one per page.
> 
> Patch 8 adds follow_pte_batch() so a contiguous same-folio run is committed
> with one refcount grab. This is the only patch whose benefit depends on folio
> size; patch 7 alone covers plain base pages.
> 
> Patches 7 and 8 carry their own benchmark tables, both measured against the
> base of the series, so the split between the two mechanisms is visible: the
> single-call walk is worth 2.3x on base pages and 2.2x on 64 kB mTHP, and
> refcount batching adds a further 5.9x on the mTHP case.
> 
> v3:
>  - split up the series into 8 much smaller patches (David & Lorenzo)
>  - shorten changelogs where they were too long (Lorenzo)
>  - fix FOLL_WRITE folio dirtying by gathering dirty bits from all PTEs

I'll try getting to this soon. As I raised previously, the whole follow_page_*
terminology is just stale, and likely we should just not add new functions that
use this terminology.

I.e., follow_page_pte_commit()

-- 
Cheers,

David