Re: [PATCH V3 5/5] ext4: make fallocate retry when err is ENOSPC

[Jan Kara]

On Mon 26-07-21 15:05:41, Wang Jianchao wrote:
> 
> 
> On 2021/7/26 11:40 AM, Guoqing Jiang wrote:
> > Hi,
> > 
> > On 7/24/21 3:41 PM, Wang Jianchao wrote:
> >> From: Wang Jianchao <wangjianchao@xxxxxxxxxxxx>
> >>
> >> The blocks may be waiting for journal commit to be freed back to
> >> mb buddy. Let fallocate wait and retry in that case.
> >>
> >> Signed-off-by: Wang Jianchao <wangjianchao@xxxxxxxxxxxx>
> >> ---
> >>   fs/ext4/extents.c | 6 +++++-
> >>   1 file changed, 5 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> >> index 92ad64b89d9b..ad0b874d3448 100644
> >> --- a/fs/ext4/extents.c
> >> +++ b/fs/ext4/extents.c
> >> @@ -4635,7 +4635,7 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> >>       struct inode *inode = file_inode(file);
> >>       loff_t new_size = 0;
> >>       unsigned int max_blocks;
> >> -    int ret = 0;
> >> +    int ret = 0, retries = 0;
> >>       int flags;
> >>       ext4_lblk_t lblk;
> >>       unsigned int blkbits = inode->i_blkbits;
> >> @@ -4656,6 +4656,7 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> >>                FALLOC_FL_INSERT_RANGE))
> >>           return -EOPNOTSUPP;
> >>   +retry:
> >>       ext4_fc_start_update(inode);
> >>         if (mode & FALLOC_FL_PUNCH_HOLE) {
> >> @@ -4722,6 +4723,9 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> >>       trace_ext4_fallocate_exit(inode, offset, max_blocks, ret);
> >>   exit:
> >>       ext4_fc_stop_update(inode);
> >> +    if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> >> +        goto retry;
> >> +
> > 
> > Not sure if it is necessary since ext4_alloc_file_blocks already retries allocate.
> 
> Yes, this patch should be get rid of.  But it is indeed helpful to fix
> the xfstest generic/371 which does concurrently write/rm and
> fallocate/rm. I'll figure out some other way to improve that

Note that the retry logic is only a heuristic. It is not guaranteed any
number of retries is enough, we just do three to not give up too easily...
Your patch effectively raised number of retries to 9 so that may have
masked the issue. But I don't think so high number of retries is a sensible
choice because that way it may take too long to return ENOSPC.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR