The patch titled
Subject: mm/gup: avoid an unnecessary allocation call for FOLL_LONGTERM cases
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-gup-avoid-an-unnecessary-allocation-call-for-foll_longterm-cases.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-gup-avoid-an-unnecessary-allocation-call-for-foll_longterm-cases.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: John Hubbard <jhubbard@xxxxxxxxxx>
Subject: mm/gup: avoid an unnecessary allocation call for FOLL_LONGTERM cases
Date: Mon, 4 Nov 2024 19:29:44 -0800
commit 53ba78de064b ("mm/gup: introduce
check_and_migrate_movable_folios()") created a new constraint on the
pin_user_pages*() API family: a potentially large internal allocation must
now occur, for FOLL_LONGTERM cases.
A user-visible consequence has now appeared: user space can no longer pin
more than 2GB of memory anymore on x86_64. That's because, on a 4KB
PAGE_SIZE system, when user space tries to (indirectly, via a device
driver that calls pin_user_pages()) pin 2GB, this requires an allocation
of a folio pointers array of MAX_PAGE_ORDER size, which is the limit for
kmalloc().
In addition to the directly visible effect described above, there is also
the problem of adding an unnecessary allocation. The **pages array
argument has already been allocated, and there is no need for a redundant
**folios array allocation in this case.
Fix this by avoiding the new allocation entirely. This is done by
referring to either the original page[i] within **pages, or to the
associated folio. Thanks to David Hildenbrand for suggesting this
approach and for providing the initial implementation (which I've tested
and adjusted slightly) as well.
Link: https://lkml.kernel.org/r/20241105032944.141488-2-jhubbard@xxxxxxxxxx
Fixes: 53ba78de064b ("mm/gup: introduce check_and_migrate_movable_folios()")
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxxx>
Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Dongwon Kim <dongwon.kim@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Junxiao Chang <junxiao.chang@xxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/gup.c | 116 +++++++++++++++++++++++++++++++++++------------------
1 file changed, 77 insertions(+), 39 deletions(-)
--- a/mm/gup.c~mm-gup-avoid-an-unnecessary-allocation-call-for-foll_longterm-cases
+++ a/mm/gup.c
@@ -2273,20 +2273,57 @@ struct page *get_dump_page(unsigned long
#endif /* CONFIG_ELF_CORE */
#ifdef CONFIG_MIGRATION
+
+/*
+ * An array of either pages or folios ("pofs"). Although it may seem tempting to
+ * avoid this complication, by simply interpreting a list of folios as a list of
+ * pages, that approach won't work in the longer term, because eventually the
+ * layouts of struct page and struct folio will become completely different.
+ * Furthermore, this pof approach avoids excessive page_folio() calls.
+ */
+struct pages_or_folios {
+ union {
+ struct page **pages;
+ struct folio **folios;
+ void **entries;
+ };
+ bool has_folios;
+ long nr_entries;
+};
+
+static struct folio *pofs_get_folio(struct pages_or_folios *pofs, long i)
+{
+ if (pofs->has_folios)
+ return pofs->folios[i];
+ return page_folio(pofs->pages[i]);
+}
+
+static void pofs_clear_entry(struct pages_or_folios *pofs, long i)
+{
+ pofs->entries[i] = NULL;
+}
+
+static void pofs_unpin(struct pages_or_folios *pofs)
+{
+ if (pofs->has_folios)
+ unpin_folios(pofs->folios, pofs->nr_entries);
+ else
+ unpin_user_pages(pofs->pages, pofs->nr_entries);
+}
+
/*
* Returns the number of collected folios. Return value is always >= 0.
*/
static unsigned long collect_longterm_unpinnable_folios(
- struct list_head *movable_folio_list,
- unsigned long nr_folios,
- struct folio **folios)
+ struct list_head *movable_folio_list,
+ struct pages_or_folios *pofs)
{
unsigned long i, collected = 0;
struct folio *prev_folio = NULL;
bool drain_allow = true;
- for (i = 0; i < nr_folios; i++) {
- struct folio *folio = folios[i];
+ for (i = 0; i < pofs->nr_entries; i++) {
+ struct folio *folio = pofs_get_folio(pofs, i);
if (folio == prev_folio)
continue;
@@ -2327,16 +2364,15 @@ static unsigned long collect_longterm_un
* Returns -EAGAIN if all folios were successfully migrated or -errno for
* failure (or partial success).
*/
-static int migrate_longterm_unpinnable_folios(
- struct list_head *movable_folio_list,
- unsigned long nr_folios,
- struct folio **folios)
+static int
+migrate_longterm_unpinnable_folios(struct list_head *movable_folio_list,
+ struct pages_or_folios *pofs)
{
int ret;
unsigned long i;
- for (i = 0; i < nr_folios; i++) {
- struct folio *folio = folios[i];
+ for (i = 0; i < pofs->nr_entries; i++) {
+ struct folio *folio = pofs_get_folio(pofs, i);
if (folio_is_device_coherent(folio)) {
/*
@@ -2344,7 +2380,7 @@ static int migrate_longterm_unpinnable_f
* convert the pin on the source folio to a normal
* reference.
*/
- folios[i] = NULL;
+ pofs_clear_entry(pofs, i);
folio_get(folio);
gup_put_folio(folio, 1, FOLL_PIN);
@@ -2363,8 +2399,8 @@ static int migrate_longterm_unpinnable_f
* calling folio_isolate_lru() which takes a reference so the
* folio won't be freed if it's migrating.
*/
- unpin_folio(folios[i]);
- folios[i] = NULL;
+ unpin_folio(pofs_get_folio(pofs, i));
+ pofs_clear_entry(pofs, i);
}
if (!list_empty(movable_folio_list)) {
@@ -2387,12 +2423,26 @@ static int migrate_longterm_unpinnable_f
return -EAGAIN;
err:
- unpin_folios(folios, nr_folios);
+ pofs_unpin(pofs);
putback_movable_pages(movable_folio_list);
return ret;
}
+static long
+check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs)
+{
+ LIST_HEAD(movable_folio_list);
+ unsigned long collected;
+
+ collected =
+ collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
+ if (!collected)
+ return 0;
+
+ return migrate_longterm_unpinnable_folios(&movable_folio_list, pofs);
+}
+
/*
* Check whether all folios are *allowed* to be pinned indefinitely (long term).
* Rather confusingly, all folios in the range are required to be pinned via
@@ -2417,16 +2467,13 @@ err:
static long check_and_migrate_movable_folios(unsigned long nr_folios,
struct folio **folios)
{
- unsigned long collected;
- LIST_HEAD(movable_folio_list);
+ struct pages_or_folios pofs = {
+ .folios = folios,
+ .has_folios = true,
+ .nr_entries = nr_folios,
+ };
- collected = collect_longterm_unpinnable_folios(&movable_folio_list,
- nr_folios, folios);
- if (!collected)
- return 0;
-
- return migrate_longterm_unpinnable_folios(&movable_folio_list,
- nr_folios, folios);
+ return check_and_migrate_movable_pages_or_folios(&pofs);
}
/*
@@ -2436,22 +2483,13 @@ static long check_and_migrate_movable_fo
static long check_and_migrate_movable_pages(unsigned long nr_pages,
struct page **pages)
{
- struct folio **folios;
- long i, ret;
-
- folios = kmalloc_array(nr_pages, sizeof(*folios), GFP_KERNEL);
- if (!folios) {
- unpin_user_pages(pages, nr_pages);
- return -ENOMEM;
- }
-
- for (i = 0; i < nr_pages; i++)
- folios[i] = page_folio(pages[i]);
+ struct pages_or_folios pofs = {
+ .pages = pages,
+ .has_folios = false,
+ .nr_entries = nr_pages,
+ };
- ret = check_and_migrate_movable_folios(nr_pages, folios);
-
- kfree(folios);
- return ret;
+ return check_and_migrate_movable_pages_or_folios(&pofs);
}
#else
static long check_and_migrate_movable_pages(unsigned long nr_pages,
_
Patches currently in -mm which might be from jhubbard@xxxxxxxxxx are
mm-gup-avoid-an-unnecessary-allocation-call-for-foll_longterm-cases.patch
kaslr-rename-physmem_end-and-physmem_end-to-direct_map_physmem_end.patch
