Re: [PATCH] fuse: fix inode rwsem regression

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 2/1/20 6:49 AM, qiwuchen55@gmail.com wrote:
> From: chenqiwu <chenqiwu@xiaomi.com>
> 
> Apparently our current rwsem code doesn't like doing the trylock, then
> lock for real scheme.  So change our direct write method to just do the
> trylock for the RWF_NOWAIT case.
> This seems to fix AIM7 regression in some scalable filesystems upto ~25%
> in some cases. Claimed in commit 942491c9e6d6 ("xfs: fix AIM7 regression")
> 
> Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
> ---
>  fs/fuse/file.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index ce71538..ac16994 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1529,7 +1529,13 @@ static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	ssize_t res;
>  
>  	/* Don't allow parallel writes to the same file */
> -	inode_lock(inode);
> +	if (iocb->ki_flags & IOCB_NOWAIT) {
> +		if (!inode_trylock(inode))
> +			return -EAGAIN;
> +	} else {
> +		inode_lock(inode);
> +	}
> +
>  	res = generic_write_checks(iocb, from);
>  	if (res > 0) {
>  		if (!is_sync_kiocb(iocb) && iocb->ki_flags & IOCB_DIRECT) {
> 


I would actually like to ask if we can do something about this lock
altogether. Replace it with a range lock?  This very lock badly hurts
fuse shared file performance and maybe I miss something, but it should
be needed only for writes/reads going into the same file?


Thanks,
Bernd