[PATCH 1/2] mm/madvise: MADV_COLLAPSE return EAGAIN when page cannot be isolated

["Zach O'Keefe" <zokeefe@google.com>]

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.

Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 mm/khugepaged.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index beaba3952dd1..5dcb2c34651e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2374,6 +2374,7 @@ 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
@@ -2456,6 +2457,7 @@ 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:
-- 
2.37.3.998.g577e59143f-goog