Re: [PATCH] ext4: fix reserved space counter leakage

On Fri, Aug 20, 2021 at 12:45:56PM -0400, Eric Whitney wrote:

* Jeffle Xu [off-list ref]:

quoted

When ext4_es_insert_delayed_block() returns error, e.g., ENOMEM,
previously reserved space is not released as the error handling,
in which case @s_dirtyclusters_counter is left over. Since this delayed
extent failes to be inserted into extent status tree, when inode is
written back, the extra @s_dirtyclusters_counter won't be subtracted and
remains there forever.

This can leads to /sys/fs/ext4/<dev>/delayed_allocation_blocks remains
non-zero even when syncfs is executed on the filesystem.

Hi:

I think the fix below looks fine.  However, this comment doesn't look right
to me.  Are you really seeing delayed_allocation_blocks values that remain
incorrectly elevated across last closes (or across file system unmounts and
remounts)?  s_dirtyclusters_counter isn't written out to stable storage -
it's an in-memory only variable that's created when a file is first opened
and destroyed on last close.

hmmm.... Let me explain a bit about this. It can be reproduced easily by fault
injection with the code modified below:

diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index 9a3a8996aacf..29dc0da5960c 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c

@@ -794,6 +794,9 @@ static int __es_insert_extent(struct inode *inode, struct extent_status *newes)
                }
        }

+       if (!(ktime_get_ns() % 3)) {
+               return -ENOMEM;
+       }
        es = ext4_es_alloc_extent(inode, newes->es_lblk, newes->es_len,
                                  newes->es_pblk);
        if (!es)

and then run a loop
while true; do dd if=/dev/zero of=aaa bs=8192 count=10000; sync; rm -rf aaa; done

After "Cannot allocate memory reported" is shown, s_dirtyclusters_counter
was already leaked. It can cause df and free space counting incorrect in
this mount.

If my understanging is correct, in priciple, we should also check with
"WARN_ON(ei->i_reserved_data_blocks)" in the inode evict path since it
should be considered as 0.

Thanks,
Gao Xiang

Eric

quoted

Fixes: 51865fda28e5 ("ext4: let ext4 maintain extent status tree")
Cc: <redacted>
Reported-by: Gao Xiang <redacted>
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
---
 fs/ext4/inode.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 82087657860b..7f15da370281 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c

@@ -1650,6 +1650,7 @@ static int ext4_insert_delayed_block(struct inode *inode, ext4_lblk_t lblk)
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 	int ret;
 	bool allocated = false;
+	bool reserved = false;
 
 	/*
 	 * If the cluster containing lblk is shared with a delayed,

@@ -1666,6 +1667,7 @@ static int ext4_insert_delayed_block(struct inode *inode, ext4_lblk_t lblk)
 		ret = ext4_da_reserve_space(inode);
 		if (ret != 0)   /* ENOSPC */
 			goto errout;
+		reserved = true;
 	} else {   /* bigalloc */
 		if (!ext4_es_scan_clu(inode, &ext4_es_is_delonly, lblk)) {
 			if (!ext4_es_scan_clu(inode,

@@ -1678,6 +1680,7 @@ static int ext4_insert_delayed_block(struct inode *inode, ext4_lblk_t lblk)
 					ret = ext4_da_reserve_space(inode);
 					if (ret != 0)   /* ENOSPC */
 						goto errout;
+					reserved = true;
 				} else {
 					allocated = true;
 				}

@@ -1688,6 +1691,8 @@ static int ext4_insert_delayed_block(struct inode *inode, ext4_lblk_t lblk)
 	}
 
 	ret = ext4_es_insert_delayed_block(inode, lblk, allocated);
+	if (ret && reserved)
+		ext4_da_release_space(inode, 1);
 
 errout:
 	return ret;

-- 
2.27.0