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...

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@nvidia.com 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@nvidia.com Fixes: 53ba78de064b ("mm/gup: introduce check_and_migrate_movable_folios()") Signed-off-by: John Hubbard jhubbard@nvidia.com Suggested-by: David Hildenbrand david@redhat.com Cc: Vivek Kasireddy vivek.kasireddy@intel.com Cc: Dave Airlie airlied@redhat.com Cc: Gerd Hoffmann kraxel@redhat.com Cc: Matthew Wilcox willy@infradead.org Cc: Christoph Hellwig hch@infradead.org Cc: Jason Gunthorpe jgg@nvidia.com Cc: Peter Xu peterx@redhat.com Cc: Arnd Bergmann arnd@arndb.de Cc: Daniel Vetter daniel.vetter@ffwll.ch Cc: Dongwon Kim dongwon.kim@intel.com Cc: Hugh Dickins hughd@google.com Cc: Junxiao Chang junxiao.chang@intel.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Oscar Salvador osalvador@suse.de Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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@nvidia.com 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