linux-kernel - Re: [PATCH 03/10] coda: honor AT_STATX_DONT

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Message-ID: <20170920200409.uksexswwko54ymg3@cs.cmu.edu>
Date:   Wed, 20 Sep 2017 16:04:09 -0400
From:   Jan Harkes <jaharkes@...cmu.edu>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 03/10] coda: honor AT_STATX_DONT_SYNC

On Wed, Sep 20, 2017 at 10:39:58AM +0200, Miklos Szeredi wrote:
> The description of this flag says "Don't sync attributes with the server".
> In other words: always use the attributes cached in the kernel and don't
> send network or local messages to refresh the attributes.

What is the use case for this AT_STATX_DONT_SYNC flag?

I'm asking because the Coda userspace client potentially has attributes
that are not cached in the kernel but can be (re-)validated without
network communication. So if we just care about avoiding network
traffic we could propagate the flag up to userspace. If we want to avoid
context switches, disk I/O and only check on what happens to be cached
in the kernel the current approach works fine.

Jan

> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> Cc: Jan Harkes <jaharkes@...cmu.edu>
> ---
>  fs/coda/inode.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/coda/inode.c b/fs/coda/inode.c
> index 6058df380cc0..734672b2cbdc 100644
> --- a/fs/coda/inode.c
> +++ b/fs/coda/inode.c
> @@ -255,7 +255,10 @@ static void coda_evict_inode(struct inode *inode)
>  int coda_getattr(const struct path *path, struct kstat *stat,
>  		 u32 request_mask, unsigned int flags)
>  {
> -	int err = coda_revalidate_inode(d_inode(path->dentry));
> +	int err = 0;
> +
> +	if (!(flags & AT_STATX_DONT_SYNC))
> +		err = coda_revalidate_inode(d_inode(path->dentry));
>  	if (!err)
>  		generic_fillattr(d_inode(path->dentry), stat);
>  	return err;
> -- 
> 2.5.5
> 
>