fs: symlink write_begin allocation context fix

With the write_begin/write_end aops, page_symlink was broken because it
could no longer pass a GFP_NOFS type mask into the point where the
allocations happened.  They are done in write_begin, which would always
assume that the filesystem can be entered from reclaim.  This bug could
cause filesystem deadlocks.

The funny thing with having a gfp_t mask there is that it doesn't really
allow the caller to arbitrarily tinker with the context in which it can be
called.  It couldn't ever be GFP_ATOMIC, for example, because it needs to
take the page lock.  The only thing any callers care about is __GFP_FS
anyway, so turn that into a single flag.

Add a new flag for write_begin, AOP_FLAG_NOFS.  Filesystems can now act on
this flag in their write_begin function.  Change __grab_cache_page to
accept a nofs argument as well, to honour that flag (while we're there,
change the name to grab_cache_page_write_begin which is more instructive
and does away with random leading underscores).

This is really a more flexible way to go in the end anyway -- if a
filesystem happens to want any extra allocations aside from the pagecache
ones in ints write_begin function, it may now use GFP_KERNEL (rather than
GFP_NOFS) for common case allocations (eg.  ocfs2_alloc_write_ctxt, for a
random example).

[kosaki.motohiro@jp.fujitsu.com: fix ubifs]
[kosaki.motohiro@jp.fujitsu.com: fix fuse]
Signed-off-by: Nick Piggin <npiggin@suse.de>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: <stable@kernel.org>		[2.6.28.x]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ Cleaned up the calling convention: just pass in the AOP flags
  untouched to the grab_cache_page_write_begin() function.  That
  just simplifies everybody, and may even allow future expansion of the
  logic.   - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 2 insertions, 3 deletions

diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
index c4bdccf976b5..5fa453b49a64 100644
--- a/fs/ext3/inode.c
+++ b/fs/ext3/inode.c
@@ -1161,7 +1161,7 @@ static int ext3_write_begin(struct file *file, struct address_space *mapping,
         to = from + len;
 
 retry:
-        page = __grab_cache_page(mapping, index);
+        page = grab_cache_page_write_begin(mapping, index, flags);
         if (!page)
                 return -ENOMEM;
         *pagep = page;
diff --git a/fs/ext3/namei.c b/fs/ext3/namei.c
index 297ea8dfac7c..1dd2abe6313e 100644
--- a/fs/ext3/namei.c
+++ b/fs/ext3/namei.c
@@ -2175,8 +2175,7 @@ retry:
                  * We have a transaction open.  All is sweetness.  It also sets
                  * i_size in generic_commit_write().
                  */
-                err = __page_symlink(inode, symname, l,
-                                mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS);
+                err = __page_symlink(inode, symname, l, 1);
                 if (err) {
                         drop_nlink(inode);
                         unlock_new_inode(inode);