From 0f3e2a2c4243695c5ac3fbccce18dc74c0250df6 Mon Sep 17 00:00:00 2001 From: Zach O'Keefe Date: Thu, 22 Sep 2022 11:46:50 -0700 Subject: 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/20220922184651.1016461-1-zokeefe@google.com Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse") Signed-off-by: Zach O'Keefe Cc: Axel Rasmussen Cc: Chris Kennelly Cc: David Hildenbrand Cc: David Rientjes Cc: Hugh Dickins Cc: James Houghton Cc: "Kirill A. Shutemov" Cc: Matthew Wilcox Cc: Miaohe Lin Cc: Minchan Kim Cc: Pasha Tatashin Cc: Peter Xu Cc: Rongwei Wang Cc: SeongJae Park Cc: Song Liu Cc: Vlastimil Babka Cc: Yang Shi Signed-off-by: Andrew Morton --- mm/khugepaged.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'mm/khugepaged.c') 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: -- cgit v1.2.3