Add a new rw_semaphore to protect page_mkwrite against truncate.  Previous
i_alloc_sem was abused for this, but it's going away in this series.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/fs/ext4/inode.c
===================================================================
--- linux-2.6.orig/fs/ext4/inode.c	2011-06-20 14:25:33.779248128 +0200
+++ linux-2.6/fs/ext4/inode.c	2011-06-20 14:27:53.025907745 +0200
@@ -5357,6 +5357,7 @@ int ext4_setattr(struct dentry *dentry,
 			if (attr->ia_size > sbi->s_bitmap_maxbytes)
 				return -EFBIG;
 		}
+		down_write(&(EXT4_I(inode)->truncate_lock));
 	}
 
 	if (S_ISREG(inode->i_mode) &&
@@ -5405,6 +5406,9 @@ int ext4_setattr(struct dentry *dentry,
 			ext4_truncate(inode);
 	}
 
+	if (attr->ia_valid & ATTR_SIZE)
+		up_write(&(EXT4_I(inode)->truncate_lock));
+
 	if (!rc) {
 		setattr_copy(inode, attr);
 		mark_inode_dirty(inode);
@@ -5850,10 +5854,10 @@ int ext4_page_mkwrite(struct vm_area_str
 	struct address_space *mapping = inode->i_mapping;
 
 	/*
-	 * Get i_alloc_sem to stop truncates messing with the inode. We cannot
-	 * get i_mutex because we are already holding mmap_sem.
+	 * Get truncate_lock to stop truncates messing with the inode. We
+	 * cannot get i_mutex because we are already holding mmap_sem.
 	 */
-	down_read(&inode->i_alloc_sem);
+	down_read(&(EXT4_I(inode)->truncate_lock));
 	size = i_size_read(inode);
 	if (page->mapping != mapping || size <= page_offset(page)
 	    || !PageUptodate(page)) {
@@ -5865,7 +5869,7 @@ int ext4_page_mkwrite(struct vm_area_str
 	lock_page(page);
 	wait_on_page_writeback(page);
 	if (PageMappedToDisk(page)) {
-		up_read(&inode->i_alloc_sem);
+		up_read(&(EXT4_I(inode)->truncate_lock));
 		return VM_FAULT_LOCKED;
 	}
 
@@ -5883,7 +5887,7 @@ int ext4_page_mkwrite(struct vm_area_str
 	if (page_has_buffers(page)) {
 		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
 					ext4_bh_unmapped)) {
-			up_read(&inode->i_alloc_sem);
+			up_read(&(EXT4_I(inode)->truncate_lock));
 			return VM_FAULT_LOCKED;
 		}
 	}
@@ -5912,11 +5916,11 @@ int ext4_page_mkwrite(struct vm_area_str
 	 */
 	lock_page(page);
 	wait_on_page_writeback(page);
-	up_read(&inode->i_alloc_sem);
+	up_read(&(EXT4_I(inode)->truncate_lock));
 	return VM_FAULT_LOCKED;
 out_unlock:
 	if (ret)
 		ret = VM_FAULT_SIGBUS;
-	up_read(&inode->i_alloc_sem);
+	up_read(&(EXT4_I(inode)->truncate_lock));
 	return ret;
 }
Index: linux-2.6/fs/ext4/super.c
===================================================================
--- linux-2.6.orig/fs/ext4/super.c	2011-06-20 14:25:33.795914793 +0200
+++ linux-2.6/fs/ext4/super.c	2011-06-20 14:27:06.269243443 +0200
@@ -871,6 +871,7 @@ static struct inode *ext4_alloc_inode(st
 	ei->i_datasync_tid = 0;
 	atomic_set(&ei->i_ioend_count, 0);
 	atomic_set(&ei->i_aiodio_unwritten, 0);
+	init_rwsem(&ei->truncate_lock);
 
 	return &ei->vfs_inode;
 }
Index: linux-2.6/fs/ext4/ext4.h
===================================================================
--- linux-2.6.orig/fs/ext4/ext4.h	2011-06-20 14:25:33.752581461 +0200
+++ linux-2.6/fs/ext4/ext4.h	2011-06-20 14:27:06.272576777 +0200
@@ -844,6 +844,9 @@ struct ext4_inode_info {
 	/* on-disk additional length */
 	__u16 i_extra_isize;
 
+	/* protect page_mkwrite against truncates */
+	struct rw_semaphore truncate_lock;
+
 #ifdef CONFIG_QUOTA
 	/* quota space reservation, managed internally by quota code */
 	qsize_t i_reserved_quota;

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