This is a note to let you know that I've just added the patch titled
mm/khugepaged: fix regression in collapse_file()
to the 6.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-khugepaged-fix-regression-in-collapse_file.patch
and it can be found in the queue-6.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From e8c716bc6812202ccf4ce0f0bad3428b794fb39c Mon Sep 17 00:00:00 2001
From: Hugh Dickins <hughd@xxxxxxxxxx>
Date: Wed, 28 Jun 2023 21:31:35 -0700
Subject: mm/khugepaged: fix regression in collapse_file()
From: Hugh Dickins <hughd@xxxxxxxxxx>
commit e8c716bc6812202ccf4ce0f0bad3428b794fb39c upstream.
There is no xas_pause(&xas) in collapse_file()'s main loop, at the points
where it does xas_unlock_irq(&xas) and then continues.
That would explain why, once two weeks ago and twice yesterday, I have
hit the VM_BUG_ON_PAGE(page != xas_load(&xas), page) since "mm/khugepaged:
fix iteration in collapse_file" removed the xas_set(&xas, index) just
before it: xas.xa_node could be left pointing to a stale node, if there
was concurrent activity on the file which transformed its xarray.
I tried inserting xas_pause()s, but then even bootup crashed on that
VM_BUG_ON_PAGE(): there appears to be a subtle "nextness" implicit in
xas_pause().
xas_next() and xas_pause() are good for use in simple loops, but not in
this one: xas_set() worked well until now, so use xas_set(&xas, index)
explicitly at the head of the loop; and change that VM_BUG_ON_PAGE() not
to need its own xas_set(), and not to interfere with the xa_state (which
would probably stop the crashes from xas_pause(), but I trust that less).
The user-visible effects of this bug (if VM_BUG_ONs are configured out)
would be data loss and data leak - potentially - though in practice I
expect it is more likely that a subsequent check (e.g. on mapping or on
nr_none) would notice an inconsistency, and just abandon the collapse.
Link: https://lore.kernel.org/linux-mm/f18e4b64-3f88-a8ab-56cc-d1f5f9c58d4@xxxxxxxxxx/
Fixes: c8a8f3b4a95a ("mm/khugepaged: fix iteration in collapse_file")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: David Stevens <stevensd@xxxxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/khugepaged.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1918,9 +1918,9 @@ static int collapse_file(struct mm_struc
}
} while (1);
- xas_set(&xas, start);
for (index = start; index < end; index++) {
- page = xas_next(&xas);
+ xas_set(&xas, index);
+ page = xas_load(&xas);
VM_BUG_ON(index != xas.xa_index);
if (is_shmem) {
@@ -1935,7 +1935,6 @@ static int collapse_file(struct mm_struc
result = SCAN_TRUNCATED;
goto xa_locked;
}
- xas_set(&xas, index + 1);
}
if (!shmem_charge(mapping->host, 1)) {
result = SCAN_FAIL;
@@ -2071,7 +2070,7 @@ static int collapse_file(struct mm_struc
xas_lock_irq(&xas);
- VM_BUG_ON_PAGE(page != xas_load(&xas), page);
+ VM_BUG_ON_PAGE(page != xa_load(xas.xa, index), page);
/*
* We control three references to the page:
Patches currently in stable-queue which might be from hughd@xxxxxxxxxx are
queue-6.4/mm-khugepaged-fix-regression-in-collapse_file.patch
