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>

1 files changed, 9 insertions, 4 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index f3e5f8944d17..f8c69273c37f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2140,19 +2140,24 @@ EXPORT_SYMBOL(generic_file_direct_write);
  * Find or create a page at the given pagecache position. Return the locked
  * page. This function is specifically for buffered writes.
  */
-struct page *__grab_cache_page(struct address_space *mapping, pgoff_t index)
+struct page *grab_cache_page_write_begin(struct address_space *mapping,
+                                        pgoff_t index, unsigned flags)
 {
         int status;
         struct page *page;
+        gfp_t gfp_notmask = 0;
+        if (flags & AOP_FLAG_NOFS)
+                gfp_notmask = __GFP_FS;
 repeat:
         page = find_lock_page(mapping, index);
         if (likely(page))
                 return page;
 
-        page = page_cache_alloc(mapping);
+        page = __page_cache_alloc(mapping_gfp_mask(mapping) & ~gfp_notmask);
         if (!page)
                 return NULL;
-        status = add_to_page_cache_lru(page, mapping, index, GFP_KERNEL);
+        status = add_to_page_cache_lru(page, mapping, index,
+                                                GFP_KERNEL & ~gfp_notmask);
         if (unlikely(status)) {
                 page_cache_release(page);
                 if (status == -EEXIST)
@@ -2161,7 +2166,7 @@ repeat:
         }
         return page;
 }
-EXPORT_SYMBOL(__grab_cache_page);
+EXPORT_SYMBOL(grab_cache_page_write_begin);
 
 static ssize_t generic_perform_write(struct file *file,
                                 struct iov_iter *i, loff_t pos)