linux-kernel - Re: [PATCH 07/11] fuse: restructure fuse

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Date:	Mon, 6 Jan 2014 17:43:19 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Maxim Patlasov <mpatlasov@...allels.com>
Cc:	dev@...allels.com, xemul@...allels.com,
	fuse-devel@...ts.sourceforge.net, bfoster@...hat.com,
	linux-kernel@...r.kernel.org, jbottomley@...allels.com,
	linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
	fengguang.wu@...el.com, devel@...nvz.org
Subject: Re: [PATCH 07/11] fuse: restructure fuse_readpage()

On Fri, Dec 20, 2013 at 06:54:40PM +0400, Maxim Patlasov wrote:
> Hi Miklos,
> 
> Sorry for delay, see please inline comments below.
> 
> On 11/12/2013 09:17 PM, Miklos Szeredi wrote:
> >On Thu, Oct 10, 2013 at 05:11:25PM +0400, Maxim Patlasov wrote:
> >>Move the code filling and sending read request to a separate function. Future
> >>patches will use it for .write_begin -- partial modification of a page
> >>requires reading the page from the storage very similarly to what fuse_readpage
> >>does.
> >>
> >>Signed-off-by: Maxim Patlasov <MPatlasov@...allels.com>
> >>---
> >>  fs/fuse/file.c |   55 +++++++++++++++++++++++++++++++++++++------------------
> >>  1 file changed, 37 insertions(+), 18 deletions(-)
> >>
> >>diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> >>index b4d4189..77eb849 100644
> >>--- a/fs/fuse/file.c
> >>+++ b/fs/fuse/file.c
> >>@@ -700,21 +700,14 @@ static void fuse_short_read(struct fuse_req *req, struct inode *inode,
> >>  	}
> >>  }
> >>-static int fuse_readpage(struct file *file, struct page *page)
> >>+static int __fuse_readpage(struct file *file, struct page *page, size_t count,
> >>+			   int *err, struct fuse_req **req_pp, u64 *attr_ver_p)
> >Signature of this helper looks really ugly.  A quick look tells me that neither
> >caller actually needs 'req'.
> 
> fuse_readpage() passes 'req' to fuse_short_read(). And the latter
> uses req->pages[] to nullify a part of request.

I don't get it.  __fuse_readpage() itself call's fuse_short_read(), not callers
of __fuse_readpage().  Or do they?

> 
> >And fuse_get_attr_version() can be moved to the
> >one caller that needs it.
> 
> Yes, it's doable. But this would make attr_version mechanism less
> efficient (under some loads): suppose the file on server was
> truncated externally, then fuse_readpage() acquires
> fc->attr_version, then some innocent write bumps fc->attr_version
> while we're waiting for fuse writeback, then fuse_read_update_size()
> would noop. In the other words, it's beneficial to keep the time
> interval between acquiring fc->attr_version and subsequent
> comparison as short as possible.

Okay, lets try to keep this the way it is.  I don't like it very much, but I
fear changing user visible behavior.

Thanks,
Miklos
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