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Date: Tue, 22 Apr 2014 10:10:11 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Dave Jones <davej@...hat.com>, Hugh Dickins <hughd@...gle.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [patch] mm: filemap: update find_get_pages_tag() to deal with shadow entries
Dave Jones reports the following crash when find_get_pages_tag() runs
into an exceptional entry:
kernel BUG at mm/filemap.c:1347!
RIP: 0010:[<ffffffffb815aeab>] [<ffffffffb815aeab>] find_get_pages_tag+0x1cb/0x220
Call Trace:
[<ffffffffb815ad16>] ? find_get_pages_tag+0x36/0x220
[<ffffffffb8168511>] pagevec_lookup_tag+0x21/0x30
[<ffffffffb81595de>] filemap_fdatawait_range+0xbe/0x1e0
[<ffffffffb8159727>] filemap_fdatawait+0x27/0x30
[<ffffffffb81f2fa4>] sync_inodes_sb+0x204/0x2a0
[<ffffffffb874d98f>] ? wait_for_completion+0xff/0x130
[<ffffffffb81fa5b0>] ? vfs_fsync+0x40/0x40
[<ffffffffb81fa5c9>] sync_inodes_one_sb+0x19/0x20
[<ffffffffb81caab2>] iterate_supers+0xb2/0x110
[<ffffffffb81fa864>] sys_sync+0x44/0xb0
[<ffffffffb875c4a9>] ia32_do_call+0x13/0x13
1343 /*
1344 * This function is never used on a shmem/tmpfs
1345 * mapping, so a swap entry won't be found here.
1346 */
1347 BUG();
After 0cd6144aadd2 ("mm + fs: prepare for non-page entries in page
cache radix trees") this comment and BUG() are out of date because
exceptional entries can now appear in all mappings - as shadows of
recently evicted pages.
However, as Hugh Dickins notes,
"it is truly surprising for a PAGECACHE_TAG_WRITEBACK (and probably
any other PAGECACHE_TAG_*) to appear on an exceptional entry.
I expect it comes down to an occasional race in RCU lookup of the
radix_tree: lacking absolute synchronization, we might sometimes
catch an exceptional entry, with the tag which really belongs with
the unexceptional entry which was there an instant before."
And indeed, not only is the tree walk lockless, the tags are also read
in chunks, one radix tree node at a time. There is plenty of time for
page reclaim to swoop in and replace a page that was already looked up
as tagged with a shadow entry.
Remove the BUG() and update the comment. While reviewing all other
lookup sites for whether they properly deal with shadow entries of
evicted pages, update all the comments and fix memcg file charge
moving to not miss shmem/tmpfs swapcache pages.
Fixes: 0cd6144aadd2 ("mm + fs: prepare for non-page entries in page cache radix trees")
Reported-by: Dave Jones <davej@...hat.com>
Signed-off-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Hugh Dickins <hughd@...gle.com>
---
mm/filemap.c | 49 ++++++++++++++++++++++++++++---------------------
mm/memcontrol.c | 20 ++++++++++++--------
mm/truncate.c | 8 --------
3 files changed, 40 insertions(+), 37 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c
index a82fbe4c9e8e..d92c437a79c4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -906,8 +906,8 @@ EXPORT_SYMBOL(page_cache_prev_hole);
* Looks up the page cache slot at @mapping & @offset. If there is a
* page cache page, it is returned with an increased refcount.
*
- * If the slot holds a shadow entry of a previously evicted page, it
- * is returned.
+ * If the slot holds a shadow entry of a previously evicted page, or a
+ * swap entry from shmem/tmpfs, it is returned.
*
* Otherwise, %NULL is returned.
*/
@@ -928,9 +928,9 @@ repeat:
if (radix_tree_deref_retry(page))
goto repeat;
/*
- * Otherwise, shmem/tmpfs must be storing a swap entry
- * here as an exceptional entry: so return it without
- * attempting to raise page count.
+ * A shadow entry of a recently evicted page,
+ * or a swap entry from shmem/tmpfs. Return
+ * it without attempting to raise page count.
*/
goto out;
}
@@ -983,8 +983,8 @@ EXPORT_SYMBOL(find_get_page);
* page cache page, it is returned locked and with an increased
* refcount.
*
- * If the slot holds a shadow entry of a previously evicted page, it
- * is returned.
+ * If the slot holds a shadow entry of a previously evicted page, or a
+ * swap entry from shmem/tmpfs, it is returned.
*
* Otherwise, %NULL is returned.
*
@@ -1099,8 +1099,8 @@ EXPORT_SYMBOL(find_or_create_page);
* with ascending indexes. There may be holes in the indices due to
* not-present pages.
*
- * Any shadow entries of evicted pages are included in the returned
- * array.
+ * Any shadow entries of evicted pages, or swap entries from
+ * shmem/tmpfs, are included in the returned array.
*
* find_get_entries() returns the number of pages and shadow entries
* which were found.
@@ -1128,9 +1128,9 @@ repeat:
if (radix_tree_deref_retry(page))
goto restart;
/*
- * Otherwise, we must be storing a swap entry
- * here as an exceptional entry: so return it
- * without attempting to raise page count.
+ * A shadow entry of a recently evicted page,
+ * or a swap entry from shmem/tmpfs. Return
+ * it without attempting to raise page count.
*/
goto export;
}
@@ -1198,9 +1198,9 @@ repeat:
goto restart;
}
/*
- * Otherwise, shmem/tmpfs must be storing a swap entry
- * here as an exceptional entry: so skip over it -
- * we only reach this from invalidate_mapping_pages().
+ * A shadow entry of a recently evicted page,
+ * or a swap entry from shmem/tmpfs. Skip
+ * over it.
*/
continue;
}
@@ -1265,9 +1265,9 @@ repeat:
goto restart;
}
/*
- * Otherwise, shmem/tmpfs must be storing a swap entry
- * here as an exceptional entry: so stop looking for
- * contiguous pages.
+ * A shadow entry of a recently evicted page,
+ * or a swap entry from shmem/tmpfs. Stop
+ * looking for contiguous pages.
*/
break;
}
@@ -1341,10 +1341,17 @@ repeat:
goto restart;
}
/*
- * This function is never used on a shmem/tmpfs
- * mapping, so a swap entry won't be found here.
+ * A shadow entry of a recently evicted page.
+ *
+ * Those entries should never be tagged, but
+ * this tree walk is lockless and the tags are
+ * looked up in bulk, one radix tree node at a
+ * time, so there is a sizable window for page
+ * reclaim to evict a page we saw tagged.
+ *
+ * Skip over it.
*/
- BUG();
+ continue;
}
if (!page_cache_get_speculative(page))
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 29501f040568..c47dffdcb246 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6686,16 +6686,20 @@ static struct page *mc_handle_file_pte(struct vm_area_struct *vma,
pgoff = pte_to_pgoff(ptent);
/* page is moved even if it's not RSS of this task(page-faulted). */
- page = find_get_page(mapping, pgoff);
-
#ifdef CONFIG_SWAP
/* shmem/tmpfs may report page out on swap: account for that too. */
- if (radix_tree_exceptional_entry(page)) {
- swp_entry_t swap = radix_to_swp_entry(page);
- if (do_swap_account)
- *entry = swap;
- page = find_get_page(swap_address_space(swap), swap.val);
- }
+ if (shmem_mapping(mapping)) {
+ page = find_get_entry(mapping, pgoff);
+ if (radix_tree_exceptional_entry(page)) {
+ swp_entry_t swp = radix_to_swp_entry(page);
+ if (do_swap_account)
+ *entry = swp;
+ page = find_get_page(swap_address_space(swp), swp.val);
+ }
+ } else
+ page = find_get_page(mapping, pgoff);
+#else
+ page = find_get_page(mapping, pgoff);
#endif
return page;
}
diff --git a/mm/truncate.c b/mm/truncate.c
index e5cc39ab0751..6a78c814bebf 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -484,14 +484,6 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
unsigned long count = 0;
int i;
- /*
- * Note: this function may get called on a shmem/tmpfs mapping:
- * pagevec_lookup() might then return 0 prematurely (because it
- * got a gangful of swap entries); but it's hardly worth worrying
- * about - it can rarely have anything to free from such a mapping
- * (most pages are dirty), and already skips over any difficulties.
- */
-
pagevec_init(&pvec, 0);
while (index <= end && pagevec_lookup_entries(&pvec, mapping, index,
min(end - index, (pgoff_t)PAGEVEC_SIZE - 1) + 1,
--
1.9.2
--
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