ext4: Make sure all the block allocation paths reserve blocks

With delayed allocation we need to make sure block are reserved before
we attempt to allocate them. Otherwise we get block allocation failure
(ENOSPC) during writepages which cannot be handled. This would mean
silent data loss (We do a printk stating data will be lost). This patch
updates the DIO and fallocate code path to do block reservation before
block allocation. This is needed to make sure parallel DIO and fallocate
request doesn't take block out of delayed reserve space.

When free blocks count go below a threshold we switch to a slow patch
which looks at other CPU's accumulated percpu counter values.
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/balloc.c

@@ -1602,6 +1602,32 @@ ext4_try_to_allocate_with_rsv(struct super_block *sb, handle_t *handle,
 	return ret;
 }
 
+int ext4_claim_free_blocks(struct ext4_sb_info *sbi,
+						ext4_fsblk_t nblocks)
+{
+	s64 free_blocks;
+	ext4_fsblk_t root_blocks = 0;
+	struct percpu_counter *fbc = &sbi->s_freeblocks_counter;
+
+	free_blocks = percpu_counter_read(fbc);
+
+	if (!capable(CAP_SYS_RESOURCE) &&
+		sbi->s_resuid != current->fsuid &&
+		(sbi->s_resgid == 0 || !in_group_p(sbi->s_resgid)))
+		root_blocks = ext4_r_blocks_count(sbi->s_es);
+
+	if (free_blocks - (nblocks + root_blocks) < EXT4_FREEBLOCKS_WATERMARK)
+		free_blocks = percpu_counter_sum(&sbi->s_freeblocks_counter);
+
+	if (free_blocks < (root_blocks + nblocks))
+		/* we don't have free space */
+		return -ENOSPC;
+
+	/* reduce fs free blocks counter */
+	percpu_counter_sub(fbc, nblocks);
+	return 0;
+}
+
 /**
  * ext4_has_free_blocks()
  * @sbi:	in-core super block structure.
@@ -1623,18 +1649,17 @@ ext4_fsblk_t ext4_has_free_blocks(struct ext4_sb_info *sbi,
 		sbi->s_resuid != current->fsuid &&
 		(sbi->s_resgid == 0 || !in_group_p(sbi->s_resgid)))
 		root_blocks = ext4_r_blocks_count(sbi->s_es);
-#ifdef CONFIG_SMP
-	if (free_blocks - root_blocks < FBC_BATCH)
-		free_blocks =
-			percpu_counter_sum(&sbi->s_freeblocks_counter);
-#endif
+
+	if (free_blocks - (nblocks + root_blocks) < EXT4_FREEBLOCKS_WATERMARK)
+		free_blocks = percpu_counter_sum_positive(&sbi->s_freeblocks_counter);
+
 	if (free_blocks <= root_blocks)
 		/* we don't have free space */
 		return 0;
 	if (free_blocks - root_blocks < nblocks)
 		return free_blocks - root_blocks;
 	return nblocks;
- }
+}
 
 
 /**
@@ -1713,14 +1738,11 @@ ext4_fsblk_t ext4_old_new_blocks(handle_t *handle, struct inode *inode,
 		/*
 		 * With delalloc we already reserved the blocks
 		 */
-		*count = ext4_has_free_blocks(sbi, *count);
-	}
-	if (*count == 0) {
-		*errp = -ENOSPC;
-		return 0;	/*return with ENOSPC error */
+		if (ext4_claim_free_blocks(sbi, *count)) {
+			*errp = -ENOSPC;
+			return 0;	/*return with ENOSPC error */
+		}
 	}
-	num = *count;
-
 	/*
 	 * Check quota for allocation of this block.
 	 */
@@ -1915,9 +1937,13 @@ ext4_fsblk_t ext4_old_new_blocks(handle_t *handle, struct inode *inode,
 	le16_add_cpu(&gdp->bg_free_blocks_count, -num);
 	gdp->bg_checksum = ext4_group_desc_csum(sbi, group_no, gdp);
 	spin_unlock(sb_bgl_lock(sbi, group_no));
-	if (!EXT4_I(inode)->i_delalloc_reserved_flag)
-		percpu_counter_sub(&sbi->s_freeblocks_counter, num);
-
+	if (!EXT4_I(inode)->i_delalloc_reserved_flag && (*count != num)) {
+		/*
+		 * we allocated less blocks than we
+		 * claimed. Add the difference back.
+		 */
+		percpu_counter_add(&sbi->s_freeblocks_counter, *count - num);
+	}
 	if (sbi->s_log_groups_per_flex) {
 		ext4_group_t flex_group = ext4_flex_group(sbi, group_no);
 		spin_lock(sb_bgl_lock(sbi, flex_group));

fs/ext4/ext4.h

@@ -983,6 +983,8 @@ extern ext4_fsblk_t ext4_new_blocks(handle_t *handle, struct inode *inode,
 					unsigned long *count, int *errp);
 extern ext4_fsblk_t ext4_old_new_blocks(handle_t *handle, struct inode *inode,
 			ext4_fsblk_t goal, unsigned long *count, int *errp);
+extern int ext4_claim_free_blocks(struct ext4_sb_info *sbi,
+						ext4_fsblk_t nblocks);
 extern ext4_fsblk_t ext4_has_free_blocks(struct ext4_sb_info *sbi,
 						ext4_fsblk_t nblocks);
 extern void ext4_free_blocks(handle_t *handle, struct inode *inode,
@@ -1207,6 +1209,17 @@ do {								\
 		__ext4_std_error((sb), __func__, (errno));	\
 } while (0)
 
+#ifdef CONFIG_SMP
+/* Each CPU can accumulate FBC_BATCH blocks in their local
+ * counters. So we need to make sure we have free blocks more
+ * than FBC_BATCH  * nr_cpu_ids. Also add a window of 4 times.
+ */
+#define EXT4_FREEBLOCKS_WATERMARK (4 * (FBC_BATCH * nr_cpu_ids))
+#else
+#define EXT4_FREEBLOCKS_WATERMARK 0
+#endif
+
+
 /*
  * Inodes and files operations
  */

fs/ext4/inode.c

@@ -1537,13 +1537,10 @@ static int ext4_da_reserve_space(struct inode *inode, int nrblocks)
 	md_needed = mdblocks - EXT4_I(inode)->i_reserved_meta_blocks;
 	total = md_needed + nrblocks;
 
-	if (ext4_has_free_blocks(sbi, total) < total) {
+	if (ext4_claim_free_blocks(sbi, total)) {
 		spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
 		return -ENOSPC;
 	}
-	/* reduce fs free blocks counter */
-	percpu_counter_sub(&sbi->s_freeblocks_counter, total);
-
 	EXT4_I(inode)->i_reserved_data_blocks += nrblocks;
 	EXT4_I(inode)->i_reserved_meta_blocks = mdblocks;
 

fs/ext4/mballoc.c

@@ -2975,9 +2975,15 @@ ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac,
 	 * at write_begin() time for delayed allocation
 	 * do not double accounting
 	 */
-	if (!(ac->ac_flags & EXT4_MB_DELALLOC_RESERVED))
-		percpu_counter_sub(&sbi->s_freeblocks_counter,
-					ac->ac_b_ex.fe_len);
+	if (!(ac->ac_flags & EXT4_MB_DELALLOC_RESERVED) &&
+			ac->ac_o_ex.fe_len != ac->ac_b_ex.fe_len) {
+		/*
+		 * we allocated less blocks than we calimed
+		 * Add the difference back
+		 */
+		percpu_counter_add(&sbi->s_freeblocks_counter,
+				ac->ac_o_ex.fe_len - ac->ac_b_ex.fe_len);
+	}
 
 	if (sbi->s_log_groups_per_flex) {
 		ext4_group_t flex_group = ext4_flex_group(sbi,
@@ -4389,14 +4395,11 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
 		/*
 		 * With delalloc we already reserved the blocks
 		 */
-		ar->len = ext4_has_free_blocks(sbi, ar->len);
-	}
-
-	if (ar->len == 0) {
-		*errp = -ENOSPC;
-		return 0;
+		if (ext4_claim_free_blocks(sbi, ar->len)) {
+			*errp = -ENOSPC;
+			return 0;
+		}
 	}
-
 	while (ar->len && DQUOT_ALLOC_BLOCK(ar->inode, ar->len)) {
 		ar->flags |= EXT4_MB_HINT_NOPREALLOC;
 		ar->len--;