Re: [PATCH 2/2] fuse: add support for no forget requests

[Josef Bacik]

On Fri, Jul 26, 2024 at 04:37:52PM +0800, yangyun wrote:
> FUSE_FORGET requests are not used if the fuse file system does not
> implement the forget operation in userspace (e.g., fuse file system
> does not cache any inodes).
> 
> However, the kernel is invisible to the userspace implementation and
> always sends FUSE_FORGET requests, which can lead to performance
> degradation because of useless contex switch and memory copy in some
> cases (e.g., many inodes are evicted from icache which was described
> in commit 07e77dca8a1f ("fuse: separate queue for FORGET requests")).
> 
> Just like 'no_interrupt' in 'struct fuse_conn', we add 'no_forget'.
> But since FUSE_FORGET request does not have a reply from userspce,
> we can not use ENOSYS to reflect the 'no_forget' assignment. So add
> the FUSE_NO_FORGET_SUPPORT init flag.
> 
> Besides, if no_forget is enabled, 'nlookup' in 'struct fuse_inode'
> does not used and its value change can be disabled which are protected
> by spin_lock to reduce lock contention.
> 
> Signed-off-by: yangyun <yangyun50@xxxxxxxxxx>
> ---
>  fs/fuse/dev.c             |  6 ++++++
>  fs/fuse/dir.c             |  4 +---
>  fs/fuse/fuse_i.h          | 24 ++++++++++++++++++++++++
>  fs/fuse/inode.c           | 10 +++++-----
>  fs/fuse/readdir.c         |  8 ++------
>  include/uapi/linux/fuse.h |  3 +++
>  6 files changed, 41 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 932356833b0d..10890db9426b 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -238,6 +238,9 @@ void fuse_queue_forget(struct fuse_conn *fc, struct fuse_forget_link *forget,
>  {
>  	struct fuse_iqueue *fiq = &fc->iq;
>  
> +	if (fc->no_forget)
> +		return;
> +
>  	forget->forget_one.nodeid = nodeid;
>  	forget->forget_one.nlookup = nlookup;
>  
> @@ -257,6 +260,9 @@ void fuse_force_forget(struct fuse_mount *fm, u64 nodeid)
>  	struct fuse_forget_in inarg;
>  	FUSE_ARGS(args);
>  
> +	if (fm->fc->no_forget)
> +		return;
> +
>  	memset(&inarg, 0, sizeof(inarg));
>  	inarg.nlookup = 1;
>  	args.opcode = FUSE_FORGET;
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 6bfb3a128658..833225ed1d4f 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -236,9 +236,7 @@ static int fuse_dentry_revalidate(struct dentry *entry, unsigned int flags)
>  				fuse_force_forget(fm, outarg.nodeid);
>  				goto invalid;
>  			}
> -			spin_lock(&fi->lock);
> -			fi->nlookup++;
> -			spin_unlock(&fi->lock);
> +			fuse_nlookup_inc_if_enabled(fm->fc, fi);
>  		}
>  		if (ret == -ENOMEM || ret == -EINTR)
>  			goto out;
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index b9a5b8ec0de5..924d6b0ad700 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -860,6 +860,9 @@ struct fuse_conn {
>  	/** Passthrough support for read/write IO */
>  	unsigned int passthrough:1;
>  
> +	/** Do not send FORGET request */
> +	unsigned int no_forget:1;
> +
>  	/** Maximum stack depth for passthrough backing files */
>  	int max_stack_depth;
>  
> @@ -1029,6 +1032,27 @@ static inline void fuse_sync_bucket_dec(struct fuse_sync_bucket *bucket)
>  	rcu_read_unlock();
>  }
>  
> +static inline void fuse_nlookup_inc_if_enabled(struct fuse_conn *fc, struct fuse_inode *fi)
> +{
> +	if (fc->no_forget)
> +		return;
> +
> +	spin_lock(&fi->lock);
> +	fi->nlookup++;
> +	spin_unlock(&fi->lock);
> +}
> +
> +static inline void fuse_nlookup_dec_if_enabled(struct fuse_conn *fc, struct fuse_inode *fi)
> +{
> +	if (fc->no_forget)
> +		return;
> +
> +	spin_lock(&fi->lock);
> +	fi->nlookup--;
> +	spin_lock(&fi->lock);
> +}

This naming scheme is overly verbose, you can simply have

fuse_inc_nlookup()
fuse_dec_nlookup()

Thanks,

Josef