[2.6.27.stable][patch 2/6] fs: fix page_mkwrite error cases in core code and btrfs

[npiggin@suse.de]

[-- Attachment #1: 56a76f8275c379ed73c8a43cfa1dfa2f5e9cfa19 --]
[-- Type: text/plain, Size: 2301 bytes --]

    fs: fix page_mkwrite error cases in core code and btrfs
    
    page_mkwrite is called with neither the page lock nor the ptl held.  This
    means a page can be concurrently truncated or invalidated out from
    underneath it.  Callers are supposed to prevent truncate races themselves,
    however previously the only thing they can do in case they hit one is to
    raise a SIGBUS.  A sigbus is wrong for the case that the page has been
    invalidated or truncated within i_size (eg.  hole punched).  Callers may
    also have to perform memory allocations in this path, where again, SIGBUS
    would be wrong.
    
    The previous patch ("mm: page_mkwrite change prototype to match fault")
    made it possible to properly specify errors.  Convert the generic buffer.c
    code and btrfs to return sane error values (in the case of page removed
    from pagecache, VM_FAULT_NOPAGE will cause the fault handler to exit
    without doing anything, and the fault will be retried properly).
    
    This fixes core code, and converts btrfs as a template/example.  All other
    filesystems defining their own page_mkwrite should be fixed in a similar
    manner.
    
    Acked-by: Chris Mason <chris.mason@oracle.com>
    Signed-off-by: Nick Piggin <npiggin@suse.de>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

---
 fs/buffer.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Index: linux-2.6/fs/buffer.c
===================================================================
--- linux-2.6.orig/fs/buffer.c
+++ linux-2.6/fs/buffer.c
@@ -2409,7 +2409,7 @@ block_page_mkwrite(struct vm_area_struct
 	struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
 	unsigned long end;
 	loff_t size;
-	int ret = -EINVAL;
+	int ret = VM_FAULT_NOPAGE; /* make the VM retry the fault */
 
 	lock_page(page);
 	size = i_size_read(inode);
@@ -2429,10 +2429,14 @@ block_page_mkwrite(struct vm_area_struct
 	if (!ret)
 		ret = block_commit_write(page, 0, end);
 
-out_unlock:
-	if (ret)
-		ret = VM_FAULT_SIGBUS;
+	if (unlikely(ret)) {
+		if (ret == -ENOMEM)
+			ret = VM_FAULT_OOM;
+		else /* -ENOSPC, -EIO, etc */
+			ret = VM_FAULT_SIGBUS;
+	}
 
+out_unlock:
 	unlock_page(page);
 	return ret;
 }