mm/khugepaged: fix regression in collapse_file()

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@google.com/
Fixes: c8a8f3b4a95a ("mm/khugepaged: fix iteration in collapse_file")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Stevens <stevensd@chromium.org>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 3beb4ad..78c8d5d 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1937,9 +1937,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
                }
        } 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) {
@@ -1954,7 +1954,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
                                                result = SCAN_TRUNCATED;
                                                goto xa_locked;
                                        }
-                                       xas_set(&xas, index + 1);
                                }
                                if (!shmem_charge(mapping->host, 1)) {
                                        result = SCAN_FAIL;
@@ -2090,7 +2089,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 
                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: