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SubjectRe: [PATCH v2 5/8] mm/gup: Accelerate thp gup even for "pages != NULL"
On 20.06.23 01:10, Peter Xu wrote:
> The acceleration of THP was done with ctx.page_mask, however it'll be
> ignored if **pages is non-NULL.
>
> The old optimization was introduced in 2013 in 240aadeedc4a ("mm:
> accelerate mm_populate() treatment of THP pages"). It didn't explain why
> we can't optimize the **pages non-NULL case. It's possible that at that
> time the major goal was for mm_populate() which should be enough back then.

In the past we had these sub-page refcounts for THP. My best guess (and
I didn't check if that was still the case in 2013) would be that it was
simpler regarding refcount handling to to do it one-subpage at a time.

But I might be just wrong.

>
> Optimize thp for all cases, by properly looping over each subpage, doing
> cache flushes, and boost refcounts / pincounts where needed in one go.
>
> This can be verified using gup_test below:
>
> # chrt -f 1 ./gup_test -m 512 -t -L -n 1024 -r 10
>
> Before: 13992.50 ( +-8.75%)
> After: 378.50 (+-69.62%)
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> mm/gup.c | 51 ++++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 44 insertions(+), 7 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 4a00d609033e..b50272012e49 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1199,16 +1199,53 @@ static long __get_user_pages(struct mm_struct *mm,
> goto out;
> }
> next_page:
> - if (pages) {
> - pages[i] = page;
> - flush_anon_page(vma, page, start);
> - flush_dcache_page(page);
> - ctx.page_mask = 0;
> - }
> -
> page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
> if (page_increm > nr_pages)
> page_increm = nr_pages;
> +
> + if (pages) {
> + struct page *subpage;
> + unsigned int j;
> +
> + /*
> + * This must be a large folio (and doesn't need to
> + * be the whole folio; it can be part of it), do
> + * the refcount work for all the subpages too.
> + *
> + * NOTE: here the page may not be the head page
> + * e.g. when start addr is not thp-size aligned.
> + * try_grab_folio() should have taken care of tail
> + * pages.
> + */
> + if (page_increm > 1) {
> + struct folio *folio;
> +
> + /*
> + * Since we already hold refcount on the
> + * large folio, this should never fail.
> + */
> + folio = try_grab_folio(page, page_increm - 1,
> + foll_flags);
> + if (WARN_ON_ONCE(!folio)) {
> + /*
> + * Release the 1st page ref if the
> + * folio is problematic, fail hard.
> + */
> + gup_put_folio(page_folio(page), 1,
> + foll_flags);
> + ret = -EFAULT;
> + goto out;
> + }
> + }
> +
> + for (j = 0; j < page_increm; j++) {
> + subpage = nth_page(page, j);
> + pages[i+j] = subpage;

Doe checkpatch like pages[i+j]? I'd have used spaces around the +.

> + flush_anon_page(vma, subpage, start + j * PAGE_SIZE);
> + flush_dcache_page(subpage);
> + }
> + }
> +
> i += page_increm;
> start += page_increm * PAGE_SIZE;
> nr_pages -= page_increm;


So, we did the first try_grab_folio() while our page was PMD-mapped
udner the PT lock and we had sufficient permissions (e.g., mapped
writable, no unsharing required). With FOLL_PIN, we incremented the
pincount.


I was wondering if something could have happened ever since we unlocked
the PT table lock and possibly PTE-mapped the THP. ... but as it's
already pinned, it cannot get shared during fork() [will stay exclusive].

So we can just take additional pins on that folio.


LGTM, although I do like the GUP-fast way of recording+ref'ing it at a
central place (see gup_huge_pmd() with record_subpages() and friends),
not after the effects.

--
Cheers,

David / dhildenb

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