fscache: Netfs function for cleanup post readpages

Currently the fscache code expect the netfs to call fscache_readpages_or_alloc
inside the aops readpages callback.  It marks all the pages in the list
provided by readahead with PG_private_2.  In the cases that the netfs fails to
read all the pages (which is legal) it ends up returning to the readahead and
triggering a BUG.  This happens because the page list still contains marked
pages.

This patch implements a simple fscache_readpages_cancel function that the netfs
should call before returning from readpages.  It will revoke the pages from the
underlying cache backend and unmark them.

The problem was originally worked out in the Ceph devel tree, but it also
occurs in CIFS.  It appears that NFS, AFS and 9P are okay as read_cache_pages()
will clean up the unprocessed pages in the case of an error.

This can be used to address the following oops:

[12410647.597278] BUG: Bad page state in process petabucket  pfn:3d504e
[12410647.597292] page:ffffea000f541380 count:0 mapcount:0 mapping:
	(null) index:0x0
[12410647.597298] page flags: 0x200000000001000(private_2)

...

[12410647.597334] Call Trace:
[12410647.597345]  [<ffffffff815523f2>] dump_stack+0x19/0x1b
[12410647.597356]  [<ffffffff8111def7>] bad_page+0xc7/0x120
[12410647.597359]  [<ffffffff8111e49e>] free_pages_prepare+0x10e/0x120
[12410647.597361]  [<ffffffff8111fc80>] free_hot_cold_page+0x40/0x170
[12410647.597363]  [<ffffffff81123507>] __put_single_page+0x27/0x30
[12410647.597365]  [<ffffffff81123df5>] put_page+0x25/0x40
[12410647.597376]  [<ffffffffa02bdcf9>] ceph_readpages+0x2e9/0x6e0 [ceph]
[12410647.597379]  [<ffffffff81122a8f>] __do_page_cache_readahead+0x1af/0x260
[12410647.597382]  [<ffffffff81122ea1>] ra_submit+0x21/0x30
[12410647.597384]  [<ffffffff81118f64>] filemap_fault+0x254/0x490
[12410647.597387]  [<ffffffff8113a74f>] __do_fault+0x6f/0x4e0
[12410647.597391]  [<ffffffff810125bd>] ? __switch_to+0x16d/0x4a0
[12410647.597395]  [<ffffffff810865ba>] ? finish_task_switch+0x5a/0xc0
[12410647.597398]  [<ffffffff8113d856>] handle_pte_fault+0xf6/0x930
[12410647.597401]  [<ffffffff81008c33>] ? pte_mfn_to_pfn+0x93/0x110
[12410647.597403]  [<ffffffff81008cce>] ? xen_pmd_val+0xe/0x10
[12410647.597405]  [<ffffffff81005469>] ? __raw_callee_save_xen_pmd_val+0x11/0x1e
[12410647.597407]  [<ffffffff8113f361>] handle_mm_fault+0x251/0x370
[12410647.597411]  [<ffffffff812b0ac4>] ? call_rwsem_down_read_failed+0x14/0x30
[12410647.597414]  [<ffffffff8155bffa>] __do_page_fault+0x1aa/0x550
[12410647.597418]  [<ffffffff8108011d>] ? up_write+0x1d/0x20
[12410647.597422]  [<ffffffff8113141c>] ? vm_mmap_pgoff+0xbc/0xe0
[12410647.597425]  [<ffffffff81143bb8>] ? SyS_mmap_pgoff+0xd8/0x240
[12410647.597427]  [<ffffffff8155c3ae>] do_page_fault+0xe/0x10
[12410647.597431]  [<ffffffff81558818>] page_fault+0x28/0x30

Signed-off-by: Milosz Tanski <milosz@adfin.com>
Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/fs/fscache/page.c b/fs/fscache/page.c
index 793e3d5..8702b73 100644
--- a/fs/fscache/page.c
+++ b/fs/fscache/page.c
@@ -701,6 +701,22 @@ nobufs:
 EXPORT_SYMBOL(__fscache_alloc_page);
 
 /*
+ * Unmark pages allocate in the readahead code path (via:
+ * fscache_readpages_or_alloc) after delegating to the base filesystem
+ */
+void __fscache_readpages_cancel(struct fscache_cookie *cookie,
+				struct list_head *pages)
+{
+	struct page *page;
+
+	list_for_each_entry(page, pages, lru) {
+		if (PageFsCache(page))
+			__fscache_uncache_page(cookie, page);
+	}
+}
+EXPORT_SYMBOL(__fscache_readpages_cancel);
+
+/*
  * release a write op reference
  */
 static void fscache_release_write_op(struct fscache_operation *_op)