mm/madvise: MADV_COLLAPSE return EAGAIN when page cannot be isolated

MADV_COLLAPSE is a best-effort request that attempts to set an actionable
errno value if the request cannot be fulfilled at the time.  EAGAIN should
be used to communicate that a resource was temporarily unavailable, but
that the user may try again immediately.

SCAN_DEL_PAGE_LRU is an internal result code used when a page cannot be
isolated from it's LRU list.  Since this, like SCAN_PAGE_LRU, is likely a
transitory state, make MADV_COLLAPSE return EAGAIN so that users know they
may reattempt the operation.

Another important scenario to consider is race with khugepaged. 
khugepaged might isolate a page while MADV_COLLAPSE is interested in it. 
Even though racing with khugepaged might mean that the memory has already
been collapsed, signalling an errno that is non-intrinsic to that memory
or arguments provided to madvise(2) lets the user know that future
attempts might (and in this case likely would) succeed, and avoids
false-negative assumptions by the user.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
Signed-off-by: Zach O'Keefe <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Chris Kennelly <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: James Houghton <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Rongwei Wang <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 2 insertions, 0 deletions

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 40fd9f7b3ed3..b3ebe90a66d9 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2372,6 +2372,7 @@ static int madvise_collapse_errno(enum scan_result r)
 	/* Resource temporary unavailable - trying again might succeed */
 	case SCAN_PAGE_LOCK:
 	case SCAN_PAGE_LRU:
+	case SCAN_DEL_PAGE_LRU:
 		return -EAGAIN;
 	/*
 	 * Other: Trying again likely not to succeed / error intrinsic to
@@ -2454,6 +2455,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
 		case SCAN_PAGE_LOCK:
 		case SCAN_PAGE_COMPOUND:
 		case SCAN_PAGE_LRU:
+		case SCAN_DEL_PAGE_LRU:
 			last_fail = result;
 			break;
 		default: