Btrfs: btrfs_page_mkwrite: Reserve space in sectorsized units

In subpagesize-blocksize scenario, if i_size occurs in a block which is not
the last block in the page, then the space to be reserved should be calculated
appropriately.
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -8802,15 +8802,28 @@ int btrfs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	loff_t size;
 	int ret;
 	int reserved = 0;
+	u64 reserved_space;
 	u64 page_start;
 	u64 page_end;
+	u64 end;
+
+	reserved_space = PAGE_CACHE_SIZE;
 
 	sb_start_pagefault(inode->i_sb);
 	page_start = page_offset(page);
 	page_end = page_start + PAGE_CACHE_SIZE - 1;
+	end = page_end;
 
+	/*
+	 * Reserving delalloc space after obtaining the page lock can lead to
+	 * deadlock. For example, if a dirty page is locked by this function
+	 * and the call to btrfs_delalloc_reserve_space() ends up triggering
+	 * dirty page write out, then the btrfs_writepage() function could
+	 * end up waiting indefinitely to get a lock on the page currently
+	 * being processed by btrfs_page_mkwrite() function.
+	 */
 	ret = btrfs_delalloc_reserve_space(inode, page_start,
-					   PAGE_CACHE_SIZE);
+					   reserved_space);
 	if (!ret) {
 		ret = file_update_time(vma->vm_file);
 		reserved = 1;
@@ -8844,7 +8857,7 @@ int btrfs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	 * we can't set the delalloc bits if there are pending ordered
 	 * extents.  Drop our locks and wait for them to finish
 	 */
-	ordered = btrfs_lookup_ordered_extent(inode, page_start);
+	ordered = btrfs_lookup_ordered_range(inode, page_start, page_end);
 	if (ordered) {
 		unlock_extent_cached(io_tree, page_start, page_end,
 				     &cached_state, GFP_NOFS);
@@ -8854,6 +8867,18 @@ int btrfs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 		goto again;
 	}
 
+	if (page->index == ((size - 1) >> PAGE_CACHE_SHIFT)) {
+		reserved_space = round_up(size - page_start, root->sectorsize);
+		if (reserved_space < PAGE_CACHE_SIZE) {
+			end = page_start + reserved_space - 1;
+			spin_lock(&BTRFS_I(inode)->lock);
+			BTRFS_I(inode)->outstanding_extents++;
+			spin_unlock(&BTRFS_I(inode)->lock);
+			btrfs_delalloc_release_space(inode, page_start,
+						PAGE_CACHE_SIZE - reserved_space);
+		}
+	}
+
 	/*
 	 * XXX - page_mkwrite gets called every time the page is dirtied, even
 	 * if it was already dirty, so for space accounting reasons we need to
@@ -8861,12 +8886,12 @@ int btrfs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	 * is probably a better way to do this, but for now keep consistent with
 	 * prepare_pages in the normal write path.
 	 */
-	clear_extent_bit(&BTRFS_I(inode)->io_tree, page_start, page_end,
+	clear_extent_bit(&BTRFS_I(inode)->io_tree, page_start, end,
 			  EXTENT_DIRTY | EXTENT_DELALLOC |
 			  EXTENT_DO_ACCOUNTING | EXTENT_DEFRAG,
 			  0, 0, &cached_state, GFP_NOFS);
 
-	ret = btrfs_set_extent_delalloc(inode, page_start, page_end,
+	ret = btrfs_set_extent_delalloc(inode, page_start, end,
 					&cached_state);
 	if (ret) {
 		unlock_extent_cached(io_tree, page_start, page_end,
@@ -8905,7 +8930,7 @@ int btrfs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	}
 	unlock_page(page);
 out:
-	btrfs_delalloc_release_space(inode, page_start, PAGE_CACHE_SIZE);
+	btrfs_delalloc_release_space(inode, page_start, reserved_space);
 out_noreserve:
 	sb_end_pagefault(inode->i_sb);
 	return ret;