shmem: shmem_charge: verify max_block is not exceeded before inode up…

…date

Patch series "userfaultfd: enable zeropage support for shmem".

These patches enable support for UFFDIO_ZEROPAGE for shared memory.

The first two patches are not strictly related to userfaultfd, they are
just minor refactoring to reduce amount of code duplication.

This patch (of 7):

Currently we update inode and shmem_inode_info before verifying that
used_blocks will not exceed max_blocks.  In case it will, we undo the
update.  Let's switch the order and move the verification of the blocks
count before the inode and shmem_inode_info update.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Rapoport <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/shmem.c

@@ -266,27 +266,26 @@ bool shmem_charge(struct inode *inode, long pages)
 
 	if (shmem_acct_block(info->flags, pages))
 		return false;
+
+	if (sbinfo->max_blocks) {
+		if (percpu_counter_compare(&sbinfo->used_blocks,
+					   sbinfo->max_blocks - pages) > 0)
+			goto unacct;
+		percpu_counter_add(&sbinfo->used_blocks, pages);
+	}
+
 	spin_lock_irqsave(&info->lock, flags);
 	info->alloced += pages;
 	inode->i_blocks += pages * BLOCKS_PER_PAGE;
 	shmem_recalc_inode(inode);
 	spin_unlock_irqrestore(&info->lock, flags);
 	inode->i_mapping->nrpages += pages;
 
-	if (!sbinfo->max_blocks)
-		return true;
-	if (percpu_counter_compare(&sbinfo->used_blocks,
-				sbinfo->max_blocks - pages) > 0) {
-		inode->i_mapping->nrpages -= pages;
-		spin_lock_irqsave(&info->lock, flags);
-		info->alloced -= pages;
-		shmem_recalc_inode(inode);
-		spin_unlock_irqrestore(&info->lock, flags);
-		shmem_unacct_blocks(info->flags, pages);
-		return false;
-	}
-	percpu_counter_add(&sbinfo->used_blocks, pages);
 	return true;
+
+unacct:
+	shmem_unacct_blocks(info->flags, pages);
+	return false;
 }
 
 void shmem_uncharge(struct inode *inode, long pages)