ext4: attempt to fix race in bigalloc code path

Currently, there exists a race between delayed allocated writes and
the writeback when bigalloc feature is in use. The race was because we
wanted to determine what blocks in a cluster are under delayed
allocation and we were using buffer_delayed(bh) check for it. But, the
writeback codepath clears this bit without any synchronization which
resulted in a race and an ext4 warning similar to:

EXT4-fs (ram1): ext4_da_update_reserve_space: ino 13, used 1 with only 0
		reserved data blocks

The race existed in two places.
(1) between ext4_find_delalloc_range() and ext4_map_blocks() when called from
    writeback code path.
(2) between ext4_find_delalloc_range() and ext4_da_get_block_prep() (where
    buffer_delayed(bh) is set.

To fix (1), this patch introduces a new buffer_head state bit -
BH_Da_Mapped.  This bit is set under the protection of
EXT4_I(inode)->i_data_sem when we have actually mapped the delayed
allocated blocks during the writeout time. We can now reliably check
for this bit inside ext4_find_delalloc_range() to determine whether
the reservation for the blocks have already been claimed or not.

To fix (2), it was necessary to set buffer_delay(bh) under the
protection of i_data_sem.  So, I extracted the very beginning of
ext4_map_blocks into a new function - ext4_da_map_blocks() - and
performed the required setting of bh_delay bit and the quota
reservation under the protection of i_data_sem.  These two fixes makes
the checking of buffer_delay(bh) and buffer_da_mapped(bh) consistent,
thus removing the race.

Tested: I was able to reproduce the problem by running 'dd' and
'fsync' in parallel. Also, xfstests sometimes used to reproduce this
race. After the fix both my test and xfstests were successful and no
race (warning message) was observed.

Google-Bug-Id: 4997027

Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1893,7 +1893,6 @@ extern int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf);
 extern qsize_t *ext4_get_reserved_space(struct inode *inode);
 extern void ext4_da_update_reserve_space(struct inode *inode,
 					int used, int quota_claim);
-extern int ext4_da_reserve_space(struct inode *inode, ext4_lblk_t lblock);
 
 /* indirect.c */
 extern int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
@@ -2300,10 +2299,14 @@ enum ext4_state_bits {
 				 * never, ever appear in a buffer_head's state
 				 * flag. See EXT4_MAP_FROM_CLUSTER to see where
 				 * this is used. */
+	BH_Da_Mapped,	/* Delayed allocated block that now has a mapping. This
+			 * flag is set when ext4_map_blocks is called on a
+			 * delayed allocated block to get its real mapping. */
 };
 
 BUFFER_FNS(Uninit, uninit)
 TAS_BUFFER_FNS(Uninit, uninit)
+BUFFER_FNS(Da_Mapped, da_mapped)
 
 /*
  * Add new method to test wether block and inode bitmaps are properly

fs/ext4/extents.c

@@ -3296,28 +3296,9 @@ static int ext4_find_delalloc_range(struct inode *inode,
 
 	while ((i >= lblk_start) && (i <= lblk_end)) {
 		page = find_get_page(mapping, index);
-		if (!page || !PageDirty(page))
+		if (!page)
 			goto nextpage;
 
-		if (PageWriteback(page)) {
-			/*
-			 * This might be a race with allocation and writeout. In
-			 * this case we just assume that the rest of the range
-			 * will eventually be written and there wont be any
-			 * delalloc blocks left.
-			 * TODO: the above assumption is troublesome, but might
-			 * work better in practice. other option could be note
-			 * somewhere that the cluster is getting written out and
-			 * detect that here.
-			 */
-			page_cache_release(page);
-			trace_ext4_find_delalloc_range(inode,
-					lblk_start, lblk_end,
-					search_hint_reverse,
-					0, i);
-			return 0;
-		}
-
 		if (!page_has_buffers(page))
 			goto nextpage;
 
@@ -3340,7 +3321,11 @@ static int ext4_find_delalloc_range(struct inode *inode,
 				continue;
 			}
 
-			if (buffer_delay(bh)) {
+			/* Check if the buffer is delayed allocated and that it
+			 * is not yet mapped. (when da-buffers are mapped during
+			 * their writeout, their da_mapped bit is set.)
+			 */
+			if (buffer_delay(bh) && !buffer_da_mapped(bh)) {
 				page_cache_release(page);
 				trace_ext4_find_delalloc_range(inode,
 						lblk_start, lblk_end,
@@ -4106,6 +4091,7 @@ got_allocated_blocks:
 			ext4_da_update_reserve_space(inode, allocated_clusters,
 							1);
 			if (reserved_clusters < allocated_clusters) {
+				struct ext4_inode_info *ei = EXT4_I(inode);
 				int reservation = allocated_clusters -
 						  reserved_clusters;
 				/*
@@ -4148,11 +4134,11 @@ got_allocated_blocks:
 				 *   remaining blocks finally gets written, we
 				 *   could claim them.
 				 */
-				while (reservation) {
-					ext4_da_reserve_space(inode,
-							      map->m_lblk);
-					reservation--;
-				}
+				dquot_reserve_block(inode,
+						EXT4_C2B(sbi, reservation));
+				spin_lock(&ei->i_block_reservation_lock);
+				ei->i_reserved_data_blocks += reservation;
+				spin_unlock(&ei->i_block_reservation_lock);
 			}
 		}
 	}

fs/ext4/inode.c

@@ -397,6 +397,49 @@ static pgoff_t ext4_num_dirty_pages(struct inode *inode, pgoff_t idx,
 	return num;
 }
 
+/*
+ * Sets the BH_Da_Mapped bit on the buffer heads corresponding to the given map.
+ */
+static void set_buffers_da_mapped(struct inode *inode,
+				   struct ext4_map_blocks *map)
+{
+	struct address_space *mapping = inode->i_mapping;
+	struct pagevec pvec;
+	int i, nr_pages;
+	pgoff_t index, end;
+
+	index = map->m_lblk >> (PAGE_CACHE_SHIFT - inode->i_blkbits);
+	end = (map->m_lblk + map->m_len - 1) >>
+		(PAGE_CACHE_SHIFT - inode->i_blkbits);
+
+	pagevec_init(&pvec, 0);
+	while (index <= end) {
+		nr_pages = pagevec_lookup(&pvec, mapping, index,
+					  min(end - index + 1,
+					      (pgoff_t)PAGEVEC_SIZE));
+		if (nr_pages == 0)
+			break;
+		for (i = 0; i < nr_pages; i++) {
+			struct page *page = pvec.pages[i];
+			struct buffer_head *bh, *head;
+
+			if (unlikely(page->mapping != mapping) ||
+			    !PageDirty(page))
+				break;
+
+			if (page_has_buffers(page)) {
+				bh = head = page_buffers(page);
+				do {
+					set_buffer_da_mapped(bh);
+					bh = bh->b_this_page;
+				} while (bh != head);
+			}
+			index++;
+		}
+		pagevec_release(&pvec);
+	}
+}
+
 /*
  * The ext4_map_blocks() function tries to look up the requested blocks,
  * and returns if the blocks are already mapped.
@@ -516,9 +559,17 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 			(flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE))
 			ext4_da_update_reserve_space(inode, retval, 1);
 	}
-	if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE)
+	if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE) {
 		ext4_clear_inode_state(inode, EXT4_STATE_DELALLOC_RESERVED);
 
+		/* If we have successfully mapped the delayed allocated blocks,
+		 * set the BH_Da_Mapped bit on them. Its important to do this
+		 * under the protection of i_data_sem.
+		 */
+		if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED)
+			set_buffers_da_mapped(inode, map);
+	}
+
 	up_write((&EXT4_I(inode)->i_data_sem));
 	if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED) {
 		int ret = check_block_validity(inode, map);
@@ -1038,7 +1089,7 @@ static int ext4_journalled_write_end(struct file *file,
 /*
  * Reserve a single cluster located at lblock
  */
-int ext4_da_reserve_space(struct inode *inode, ext4_lblk_t lblock)
+static int ext4_da_reserve_space(struct inode *inode, ext4_lblk_t lblock)
 {
 	int retries = 0;
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
@@ -1153,6 +1204,7 @@ static void ext4_da_page_release_reservation(struct page *page,
 		if ((offset <= curr_off) && (buffer_delay(bh))) {
 			to_release++;
 			clear_buffer_delay(bh);
+			clear_buffer_da_mapped(bh);
 		}
 		curr_off = next_off;
 	} while ((bh = bh->b_this_page) != head);
@@ -1271,6 +1323,8 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
 						clear_buffer_delay(bh);
 						bh->b_blocknr = pblock;
 					}
+					if (buffer_da_mapped(bh))
+						clear_buffer_da_mapped(bh);
 					if (buffer_unwritten(bh) ||
 					    buffer_mapped(bh))
 						BUG_ON(bh->b_blocknr != pblock);
@@ -1603,6 +1657,66 @@ static int ext4_bh_delay_or_unwritten(handle_t *handle, struct buffer_head *bh)
 	return (buffer_delay(bh) || buffer_unwritten(bh)) && buffer_dirty(bh);
 }
 
+/*
+ * This function is grabs code from the very beginning of
+ * ext4_map_blocks, but assumes that the caller is from delayed write
+ * time. This function looks up the requested blocks and sets the
+ * buffer delay bit under the protection of i_data_sem.
+ */
+static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
+			      struct ext4_map_blocks *map,
+			      struct buffer_head *bh)
+{
+	int retval;
+	sector_t invalid_block = ~((sector_t) 0xffff);
+
+	if (invalid_block < ext4_blocks_count(EXT4_SB(inode->i_sb)->s_es))
+		invalid_block = ~0;
+
+	map->m_flags = 0;
+	ext_debug("ext4_da_map_blocks(): inode %lu, max_blocks %u,"
+		  "logical block %lu\n", inode->i_ino, map->m_len,
+		  (unsigned long) map->m_lblk);
+	/*
+	 * Try to see if we can get the block without requesting a new
+	 * file system block.
+	 */
+	down_read((&EXT4_I(inode)->i_data_sem));
+	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
+		retval = ext4_ext_map_blocks(NULL, inode, map, 0);
+	else
+		retval = ext4_ind_map_blocks(NULL, inode, map, 0);
+
+	if (retval == 0) {
+		/*
+		 * XXX: __block_prepare_write() unmaps passed block,
+		 * is it OK?
+		 */
+		/* If the block was allocated from previously allocated cluster,
+		 * then we dont need to reserve it again. */
+		if (!(map->m_flags & EXT4_MAP_FROM_CLUSTER)) {
+			retval = ext4_da_reserve_space(inode, iblock);
+			if (retval)
+				/* not enough space to reserve */
+				goto out_unlock;
+		}
+
+		/* Clear EXT4_MAP_FROM_CLUSTER flag since its purpose is served
+		 * and it should not appear on the bh->b_state.
+		 */
+		map->m_flags &= ~EXT4_MAP_FROM_CLUSTER;
+
+		map_bh(bh, inode->i_sb, invalid_block);
+		set_buffer_new(bh);
+		set_buffer_delay(bh);
+	}
+
+out_unlock:
+	up_read((&EXT4_I(inode)->i_data_sem));
+
+	return retval;
+}
+
 /*
  * This is a special get_blocks_t callback which is used by
  * ext4_da_write_begin().  It will either return mapped block or
@@ -1620,10 +1734,6 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
 {
 	struct ext4_map_blocks map;
 	int ret = 0;
-	sector_t invalid_block = ~((sector_t) 0xffff);
-
-	if (invalid_block < ext4_blocks_count(EXT4_SB(inode->i_sb)->s_es))
-		invalid_block = ~0;
 
 	BUG_ON(create == 0);
 	BUG_ON(bh->b_size != inode->i_sb->s_blocksize);
@@ -1636,29 +1746,9 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
 	 * preallocated blocks are unmapped but should treated
 	 * the same as allocated blocks.
 	 */
-	ret = ext4_map_blocks(NULL, inode, &map, 0);
-	if (ret < 0)
+	ret = ext4_da_map_blocks(inode, iblock, &map, bh);
+	if (ret <= 0)
 		return ret;
-	if (ret == 0) {
-		if (buffer_delay(bh))
-			return 0; /* Not sure this could or should happen */
-		/*
-		 * XXX: __block_write_begin() unmaps passed block, is it OK?
-		 */
-		/* If the block was allocated from previously allocated cluster,
-		 * then we dont need to reserve it again. */
-		if (!(map.m_flags & EXT4_MAP_FROM_CLUSTER)) {
-			ret = ext4_da_reserve_space(inode, iblock);
-			if (ret)
-				/* not enough space to reserve */
-				return ret;
-		}
-
-		map_bh(bh, inode->i_sb, invalid_block);
-		set_buffer_new(bh);
-		set_buffer_delay(bh);
-		return 0;
-	}
 
 	map_bh(bh, inode->i_sb, map.m_pblk);
 	bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags;