linux-kernel - Re: [PATCH 6/7] mm/gup: Accelerate thp gup even for "pages != NULL"

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Date:   Mon, 19 Jun 2023 16:24:31 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 6/7] mm/gup: Accelerate thp gup even for "pages != NULL"

On Mon, Jun 19, 2023 at 03:37:30PM -0400, Peter Xu wrote:
> Here what I can do is still guard this try_grab_folio() and fail the GUP if
> for any reason it failed.  Perhaps then it means I'll also keep that one
> untouched in hugetlb_follow_page_mask() too.  But I suppose keeping the
> WARN_ON_ONCE() seems still proper.

Here's the outcome that I plan to post in the new version, taking care of
try_grab_folio() failures even if it happens, meanwhile remove the
compound_head() redundancy on the page.

__get_user_pages():
...
===8<===
			/*
			 * 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;
				}
			}
===8<===

Thanks,

-- 
Peter Xu