linux-kernel - [PATCH v3] gup: optimize longterm pin_user

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Message-ID: <20250605033430.83142-1-lizhe.67@bytedance.com>
Date: Thu,  5 Jun 2025 11:34:30 +0800
From: lizhe.67@...edance.com
To: akpm@...ux-foundation.org,
	david@...hat.com,
	jgg@...pe.ca,
	jhubbard@...dia.com,
	peterx@...hat.com
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	dev.jain@....com,
	muchun.song@...ux.dev,
	lizhe.67@...edance.com
Subject: [PATCH v3] gup: optimize longterm pin_user_pages() for large folio

From: Li Zhe <lizhe.67@...edance.com>

In the current implementation of the longterm pin_user_pages() function,
we invoke the collect_longterm_unpinnable_folios() function. This function
iterates through the list to check whether each folio belongs to the
"longterm_unpinnabled" category. The folios in this list essentially
correspond to a contiguous region of user-space addresses, with each folio
representing a physical address in increments of PAGESIZE. If this
user-space address range is mapped with large folio, we can optimize the
performance of function collect_longterm_unpinnable_folios() by reducing
the using of READ_ONCE() invoked in
pofs_get_folio()->page_folio()->_compound_head(). Also, we can simplify
the logic of collect_longterm_unpinnable_folios(). Instead of comparing
with prev_folio after calling pofs_get_folio(), we can check whether the
next page is within the same folio.

The performance test results, based on v6.15, obtained through the
gup_test tool from the kernel source tree are as follows. We achieve an
improvement of over 66% for large folio with pagesize=2M. For small folio,
we have only observed a very slight degradation in performance.

Without this patch:

    [root@...alhost ~] ./gup_test -HL -m 8192 -n 512
    TAP version 13
    1..1
    # PIN_LONGTERM_BENCHMARK: Time: get:14391 put:10858 us#
    ok 1 ioctl status 0
    # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
    [root@...alhost ~]# ./gup_test -LT -m 8192 -n 512
    TAP version 13
    1..1
    # PIN_LONGTERM_BENCHMARK: Time: get:130538 put:31676 us#
    ok 1 ioctl status 0
    # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

With this patch:

    [root@...alhost ~] ./gup_test -HL -m 8192 -n 512
    TAP version 13
    1..1
    # PIN_LONGTERM_BENCHMARK: Time: get:4867 put:10516 us#
    ok 1 ioctl status 0
    # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
    [root@...alhost ~]# ./gup_test -LT -m 8192 -n 512
    TAP version 13
    1..1
    # PIN_LONGTERM_BENCHMARK: Time: get:131798 put:31328 us#
    ok 1 ioctl status 0
    # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Li Zhe <lizhe.67@...edance.com>
---
Changelogs:

v2->v3:
- Update performance test data based on v6.15.
- Refine the description of the optimization approach in commit message.
- Fix some issues of code formatting.
- Fine-tune the conditions for entering the optimization path.

v1->v2:
- Modify some unreliable code.
- Update performance test data.

v2 patch: https://lore.kernel.org/all/20250604031536.9053-1-lizhe.67@bytedance.com/
v1 patch: https://lore.kernel.org/all/20250530092351.32709-1-lizhe.67@bytedance.com/

 mm/gup.c | 37 +++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 84461d384ae2..9fbe3592b5fc 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2317,6 +2317,31 @@ static void pofs_unpin(struct pages_or_folios *pofs)
 		unpin_user_pages(pofs->pages, pofs->nr_entries);
 }
 
+static struct folio *pofs_next_folio(struct folio *folio,
+				struct pages_or_folios *pofs, long *index_ptr)
+{
+	long i = *index_ptr + 1;
+
+	if (!pofs->has_folios && folio_test_large(folio)) {
+		const unsigned long start_pfn = folio_pfn(folio);
+		const unsigned long end_pfn = start_pfn + folio_nr_pages(folio);
+
+		for (; i < pofs->nr_entries; i++) {
+			unsigned long pfn = page_to_pfn(pofs->pages[i]);
+
+			/* Is this page part of this folio? */
+			if (pfn < start_pfn || pfn >= end_pfn)
+				break;
+		}
+	}
+
+	if (unlikely(i == pofs->nr_entries))
+		return NULL;
+	*index_ptr = i;
+
+	return pofs_get_folio(pofs, i);
+}
+
 /*
  * Returns the number of collected folios. Return value is always >= 0.
  */
@@ -2324,16 +2349,12 @@ static void collect_longterm_unpinnable_folios(
 		struct list_head *movable_folio_list,
 		struct pages_or_folios *pofs)
 {
-	struct folio *prev_folio = NULL;
 	bool drain_allow = true;
-	unsigned long i;
-
-	for (i = 0; i < pofs->nr_entries; i++) {
-		struct folio *folio = pofs_get_folio(pofs, i);
+	struct folio *folio;
+	long i = 0;
 
-		if (folio == prev_folio)
-			continue;
-		prev_folio = folio;
+	for (folio = pofs_get_folio(pofs, i); folio;
+			folio = pofs_next_folio(folio, pofs, &i)) {
 
 		if (folio_is_longterm_pinnable(folio))
 			continue;
-- 
2.20.1