From ae63c898f4004bbc7d212f4adcb3bb14852c30d6 Mon Sep 17 00:00:00 2001
From: Zach O'Keefe <zokeefe@google.com>
Date: Tue, 24 Jan 2023 17:57:37 -0800
Subject: [PATCH] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount
During collapse, in a few places we check to see if a given small page has
any unaccounted references. If the refcount on the page doesn't match our
expectations, it must be there is an unknown user concurrently interested
in the page, and so it's not safe to move the contents elsewhere.
However, the unaccounted pins are likely an ephemeral state.
In this situation, MADV_COLLAPSE returns -EINVAL when it should return
-EAGAIN. This could cause userspace to conclude that the syscall
failed, when it in fact could succeed by retrying.
Link: https://lkml.kernel.org/r/20230125015738.912924-1-zokeefe@google.com
Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Reported-by: Hugh Dickins <hughd@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/khugepaged.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 90acfea40c13c..a26a28e3738c1 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2611,6 +2611,7 @@ static int madvise_collapse_errno(enum scan_result r)
case SCAN_CGROUP_CHARGE_FAIL:
return -EBUSY;
/* Resource temporary unavailable - trying again might succeed */
+ case SCAN_PAGE_COUNT:
case SCAN_PAGE_LOCK:
case SCAN_PAGE_LRU:
case SCAN_DEL_PAGE_LRU:
--
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