mm: allow GUP to fail instead of waiting on a page

GUP user may want to try to acquire a reference to a page if it is already
in memory, but not if IO, to bring it in, is needed.  For example KVM may
tell vcpu to schedule another guest process if current one is trying to
access swapped out page.  Meanwhile, the page will be swapped in and the
guest process, that depends on it, will be able to run again.

This patch adds FAULT_FLAG_RETRY_NOWAIT (suggested by Linus) and
FOLL_NOWAIT follow_page flags.  FAULT_FLAG_RETRY_NOWAIT, when used in
conjunction with VM_FAULT_ALLOW_RETRY, indicates to handle_mm_fault that
it shouldn't drop mmap_sem and wait on a page, but return VM_FAULT_RETRY
instead.

[akpm@linux-foundation.org: improve FOLL_NOWAIT comment]
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1f82adc85352..901435e3a9a9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -151,6 +151,7 @@ extern pgprot_t protection_map[16];
 #define FAULT_FLAG_NONLINEAR    0x02    /* Fault was via a nonlinear mapping */
 #define FAULT_FLAG_MKWRITE      0x04    /* Fault was mkwrite of existing pte */
 #define FAULT_FLAG_ALLOW_RETRY  0x08    /* Retry fault if blocking */
+#define FAULT_FLAG_RETRY_NOWAIT 0x10    /* Don't drop mmap_sem and wait when retrying */
 
 /*
  * This interface is used by x86 PAT code to identify a pfn mapping that is
@@ -1545,6 +1546,8 @@ struct page *follow_page(struct vm_area_struct *, unsigned long address,
 #define FOLL_GET        0x04    /* do get_page on page */
 #define FOLL_DUMP       0x08    /* give error on hole if it would be zero */
 #define FOLL_FORCE      0x10    /* get_user_pages read/write w/o permission */
+#define FOLL_NOWAIT     0x20    /* if a disk transfer is needed, start the IO
+                                 * and return without waiting upon it */
 #define FOLL_MLOCK      0x40    /* mark page as mlocked */
 #define FOLL_SPLIT      0x80    /* don't return transhuge pages, split them */
 #define FOLL_HWPOISON   0x100   /* check page is hwpoisoned */