From: Hugh Dickins <hugh@veritas.com>
Extend use of that SHMEM_PAGEIN flag to where shmem_getpage adds a page
to the cache. It couldn't have caused a BUG_ON(inode->i_blocks), but if
i_size is reduced (from another cpu) the instant after shmem_swp_alloc
checks it, shmem_getpage could insert a page into the cache just after
truncate_inode_pages has passed through cleaning it, leaving stale data
(which may mysteriously reappear if the file is later extended).
Easily fixed for tmpfs, using the mechanism just added for swapoff; and
probably more important there, since its read from swap can insert non-0
data. But is there not a similar issue, a tiny window, in filemap.c?
if truncate_inode_pages comes in between checking i_size and adding new
page to cache. Not worth getting excited, but something to beware of.
25-akpm/mm/shmem.c | 6 +++++-
1 files changed, 5 insertions(+), 1 deletion(-)
diff -puN mm/shmem.c~tmpfs-04-getpage-truncate-race mm/shmem.c
--- 25/mm/shmem.c~tmpfs-04-getpage-truncate-race Wed Oct 15 12:18:26 2003
+++ 25-akpm/mm/shmem.c Wed Oct 15 12:18:26 2003
@@ -52,7 +52,7 @@
#define VM_ACCT(size) (PAGE_CACHE_ALIGN(size) >> PAGE_SHIFT)
-/* info->flags needs a VM_flag to handle swapoff/truncate races efficiently */
+/* info->flags needs a VM_flag to handle pagein/truncate races efficiently */
#define SHMEM_PAGEIN VM_READ
/* Pretend that each entry is of this size in directory's i_size */
@@ -498,6 +498,8 @@ done2:
* Call truncate_inode_pages again: racing shmem_unuse_inode
* may have swizzled a page in from swap since vmtruncate or
* generic_delete_inode did it, before we lowered next_index.
+ * Also, though shmem_getpage checks i_size before adding to
+ * cache, no recheck after: so fix the narrow window there too.
*/
spin_unlock(&info->lock);
truncate_inode_pages(inode->i_mapping, inode->i_size);
@@ -863,6 +865,7 @@ repeat:
swap_free(swap);
} else if (!(error = move_from_swap_cache(
swappage, idx, mapping))) {
+ info->flags |= SHMEM_PAGEIN;
shmem_swp_set(info, entry, 0);
shmem_swp_unmap(entry);
spin_unlock(&info->lock);
@@ -932,6 +935,7 @@ repeat:
goto failed;
goto repeat;
}
+ info->flags |= SHMEM_PAGEIN;
}
info->alloced++;
_