ext4: avoid ext4_error()'s caused by ENOMEM in the truncate path

We can't fail in the truncate path without requiring an fsck.
Add work around for this by using a combination of retry loops
and the __GFP_NOFAIL flag.

From: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Anna Pendleton <pendleton@google.com>
Reviewed-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20200507175028.15061-1-pendleton@google.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -630,6 +630,7 @@ enum {
  */
 #define EXT4_EX_NOCACHE				0x40000000
 #define EXT4_EX_FORCE_CACHE			0x20000000
+#define EXT4_EX_NOFAIL				0x10000000
 
 /*
  * Flags used by ext4_free_blocks

fs/ext4/extents.c

@@ -297,11 +297,14 @@ ext4_force_split_extent_at(handle_t *handle, struct inode *inode,
 {
 	struct ext4_ext_path *path = *ppath;
 	int unwritten = ext4_ext_is_unwritten(path[path->p_depth].p_ext);
+	int flags = EXT4_EX_NOCACHE | EXT4_GET_BLOCKS_PRE_IO;
+
+	if (nofail)
+		flags |= EXT4_GET_BLOCKS_METADATA_NOFAIL | EXT4_EX_NOFAIL;
 
 	return ext4_split_extent_at(handle, inode, ppath, lblk, unwritten ?
 			EXT4_EXT_MARK_UNWRIT1|EXT4_EXT_MARK_UNWRIT2 : 0,
-			EXT4_EX_NOCACHE | EXT4_GET_BLOCKS_PRE_IO |
-			(nofail ? EXT4_GET_BLOCKS_METADATA_NOFAIL:0));
+			flags);
 }
 
 static int
@@ -487,8 +490,12 @@ __read_extent_tree_block(const char *function, unsigned int line,
 {
 	struct buffer_head		*bh;
 	int				err;
+	gfp_t				gfp_flags = __GFP_MOVABLE | GFP_NOFS;
+
+	if (flags & EXT4_EX_NOFAIL)
+		gfp_flags |= __GFP_NOFAIL;
 
-	bh = sb_getblk_gfp(inode->i_sb, pblk, __GFP_MOVABLE | GFP_NOFS);
+	bh = sb_getblk_gfp(inode->i_sb, pblk, gfp_flags);
 	if (unlikely(!bh))
 		return ERR_PTR(-ENOMEM);
 
@@ -837,6 +844,10 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
 	struct ext4_ext_path *path = orig_path ? *orig_path : NULL;
 	short int depth, i, ppos = 0;
 	int ret;
+	gfp_t gfp_flags = GFP_NOFS;
+
+	if (flags & EXT4_EX_NOFAIL)
+		gfp_flags |= __GFP_NOFAIL;
 
 	eh = ext_inode_hdr(inode);
 	depth = ext_depth(inode);
@@ -857,7 +868,7 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
 	if (!path) {
 		/* account possible depth increase */
 		path = kcalloc(depth + 2, sizeof(struct ext4_ext_path),
-				GFP_NOFS);
+				gfp_flags);
 		if (unlikely(!path))
 			return ERR_PTR(-ENOMEM);
 		path[0].p_maxdepth = depth + 1;
@@ -1007,9 +1018,13 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
 	ext4_fsblk_t newblock, oldblock;
 	__le32 border;
 	ext4_fsblk_t *ablocks = NULL; /* array of allocated blocks */
+	gfp_t gfp_flags = GFP_NOFS;
 	int err = 0;
 	size_t ext_size = 0;
 
+	if (flags & EXT4_EX_NOFAIL)
+		gfp_flags |= __GFP_NOFAIL;
+
 	/* make decision: where to split? */
 	/* FIXME: now decision is simplest: at current extent */
 
@@ -1043,7 +1058,7 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
 	 * We need this to handle errors and free blocks
 	 * upon them.
 	 */
-	ablocks = kcalloc(depth, sizeof(ext4_fsblk_t), GFP_NOFS);
+	ablocks = kcalloc(depth, sizeof(ext4_fsblk_t), gfp_flags);
 	if (!ablocks)
 		return -ENOMEM;
 
@@ -2019,7 +2034,7 @@ int ext4_ext_insert_extent(handle_t *handle, struct inode *inode,
 	if (next != EXT_MAX_BLOCKS) {
 		ext_debug("next leaf block - %u\n", next);
 		BUG_ON(npath != NULL);
-		npath = ext4_find_extent(inode, next, NULL, 0);
+		npath = ext4_find_extent(inode, next, NULL, gb_flags);
 		if (IS_ERR(npath))
 			return PTR_ERR(npath);
 		BUG_ON(npath->p_depth != path->p_depth);
@@ -2792,7 +2807,8 @@ int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
 		ext4_fsblk_t pblk;
 
 		/* find extent for or closest extent to this block */
-		path = ext4_find_extent(inode, end, NULL, EXT4_EX_NOCACHE);
+		path = ext4_find_extent(inode, end, NULL,
+					EXT4_EX_NOCACHE | EXT4_EX_NOFAIL);
 		if (IS_ERR(path)) {
 			ext4_journal_stop(handle);
 			return PTR_ERR(path);
@@ -2878,7 +2894,7 @@ int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
 				le16_to_cpu(path[k].p_hdr->eh_entries)+1;
 	} else {
 		path = kcalloc(depth + 1, sizeof(struct ext4_ext_path),
-			       GFP_NOFS);
+			       GFP_NOFS | __GFP_NOFAIL);
 		if (path == NULL) {
 			ext4_journal_stop(handle);
 			return -ENOMEM;
@@ -3303,7 +3319,7 @@ static int ext4_split_extent(handle_t *handle,
 	 * Update path is required because previous ext4_split_extent_at() may
 	 * result in split of original leaf or extent zeroout.
 	 */
-	path = ext4_find_extent(inode, map->m_lblk, ppath, 0);
+	path = ext4_find_extent(inode, map->m_lblk, ppath, flags);
 	if (IS_ERR(path))
 		return PTR_ERR(path);
 	depth = ext_depth(inode);
@@ -4365,7 +4381,14 @@ int ext4_ext_truncate(handle_t *handle, struct inode *inode)
 	}
 	if (err)
 		return err;
-	return ext4_ext_remove_space(inode, last_block, EXT_MAX_BLOCKS - 1);
+retry_remove_space:
+	err = ext4_ext_remove_space(inode, last_block, EXT_MAX_BLOCKS - 1);
+	if (err == -ENOMEM) {
+		cond_resched();
+		congestion_wait(BLK_RW_ASYNC, HZ/50);
+		goto retry_remove_space;
+	}
+	return err;
 }
 
 static int ext4_alloc_file_blocks(struct file *file, ext4_lblk_t offset,