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/btrfs/inode.c

@@ -4307,10 +4307,15 @@ int btrfs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	u64 page_end;
 
 	ret = btrfs_check_data_free_space(root, inode, PAGE_CACHE_SIZE);
-	if (ret)
+	if (ret) {
+		if (ret == -ENOMEM)
+			ret = VM_FAULT_OOM;
+		else /* -ENOSPC, -EIO, etc */
+			ret = VM_FAULT_SIGBUS;
 		goto out;
+	}
 
-	ret = -EINVAL;
+	ret = VM_FAULT_NOPAGE; /* make the VM retry the fault */
 again:
 	lock_page(page);
 	size = i_size_read(inode);
@@ -4363,8 +4368,6 @@ int btrfs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 out_unlock:
 	unlock_page(page);
 out:
-	if (ret)
-		ret = VM_FAULT_SIGBUS;
 	return ret;
 }
 

fs/buffer.c

@@ -2320,7 +2320,7 @@ block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 	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);
@@ -2340,10 +2340,14 @@ block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 	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;
 }