+ mm-madv_collapse-set-eagain-on-unexpected-page-refcount.patch added to mm-hotfixes-unstable branch

+ mm-madv_collapse-set-eagain-on-unexpected-page-refcount.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-madv_collapse-set-eagain-on-unexpected-page-refcount.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-madv_collapse-set-eagain-on-unexpected-page-refcount.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "Zach O'Keefe" <zokeefe@google.com>
Subject: mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount
Date: Tue, 24 Jan 2023 17:57:37 -0800

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 such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that
collapse may succeed on retry.

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(+)

--- a/mm/khugepaged.c~mm-madv_collapse-set-eagain-on-unexpected-page-refcount
+++ a/mm/khugepaged.c
@@ -2611,6 +2611,7 @@ static int madvise_collapse_errno(enum s
 	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:
_

Patches currently in -mm which might be from zokeefe@google.com are

mm-madv_collapse-set-eagain-on-unexpected-page-refcount.patch