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

[Peter Xu]

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.

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@xxxxxxxxxx>
---
 mm/gup.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index a2d1b3c4b104..cdabc8ea783b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1210,16 +1210,38 @@ 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.
+			 * Since we already hold refcount on the head page,
+			 * it should never fail.
+			 *
+			 * NOTE: here the page may not be the head page
+			 * e.g. when start addr is not thp-size aligned.
+			 */
+			if (page_increm > 1)
+				WARN_ON_ONCE(
+				    try_grab_folio(compound_head(page),
+						   page_increm - 1,
+						   foll_flags) == NULL);
+
+			for (j = 0; j < page_increm; j++) {
+				subpage = nth_page(page, j);
+				pages[i+j] = subpage;
+				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;
-- 
2.40.1