[PATCH v1 1/1] mm/gup: skip pinnable check for refs==1

[Will McVicker <willmcvicker@google.com>]

This fixes commit 54d516b1d62f ("mm/gup: small refactoring: simplify
try_grab_page()") which refactors try_grab_page() to call
try_grab_compound_head() with refs=1. The refactor commit is causing
pin_user_pages() to return -ENOMEM when we try to pin one user page that
is migratable and not in the movable zone. Previously, try_grab_page()
didn't check if the page was pinnable for FOLL_PIN. To match the same
functionality, this fix adds the check `refs > 1 &&` to skip the call to
is_pinnable_page().

This issue is reproducible with the Pixel 6 on the 5.15 LTS kernel. Here
is the call stack to reproduce the -ENOMEM error:

Call trace:
        : dump_backtrace.cfi_jt+0x0/0x8
        : show_stack+0x1c/0x2c
        : dump_stack_lvl+0x68/0x98
        : try_grab_compound_head+0x298/0x3c4
        : follow_page_pte+0x1dc/0x330
        : follow_page_mask+0x174/0x340
        : __get_user_pages+0x158/0x34c
        : __gup_longterm_locked+0xfc/0x194
        : __gup_longterm_unlocked+0x80/0xf4
        : internal_get_user_pages_fast+0xf0/0x15c
        : pin_user_pages_fast+0x24/0x40
        : edgetpu_device_group_map+0x130/0x584 [abrolhos]
        : edgetpu_ioctl_map_buffer+0x110/0x3b4 [abrolhos]
        : edgetpu_ioctl+0x238/0x408 [abrolhos]
        : edgetpu_fs_ioctl+0x14/0x24 [abrolhos]

Fixes: 54d516b1d62f ("mm/gup: small refactoring: simplify try_grab_page()")
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Minchan Kim <minchan@google.com>
Signed-off-by: Will McVicker <willmcvicker@google.com>
---
 mm/gup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index f0af462ac1e2..0509c49c46a3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -135,7 +135,7 @@ struct page *try_grab_compound_head(struct page *page,
 		 * right zone, so fail and let the caller fall back to the slow
 		 * path.
 		 */
-		if (unlikely((flags & FOLL_LONGTERM) &&
+		if (refs > 1 && unlikely((flags & FOLL_LONGTERM) &&
 			     !is_pinnable_page(page)))
 			return NULL;
 
-- 
2.35.0.rc2.247.g8bbb082509-goog