linux-kernel - Re: [PATCH] ceph: Update the pages in fscache in writepages() path

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Date:	Tue, 5 Nov 2013 12:30:05 -0500
From:	Milosz Tanski <milosz@...in.com>
To:	Li Wang <liwang@...ntukylin.com>
Cc:	ceph-devel <ceph-devel@...r.kernel.org>,
	"linux-cachefs@...hat.com" <linux-cachefs@...hat.com>,
	Sage Weil <sage@...tank.com>, linux-kernel@...r.kernel.org,
	Min Chen <minchen@...ntukylin.com>,
	Yunchuan Wen <yunchuanwen@...ntukylin.com>
Subject: Re: [PATCH] ceph: Update the pages in fscache in writepages() path

Li,

First, sorry for the late reply on this.

Currently fscache is only supported for files that are open in read
only mode. I originally was going to let fscache cache in the write
path as well as long as the file was open in with O_LAZY. I abandoned
that idea. When a user opens the file in O_LAZY we can cache things
locally with the assumption that the user will care of the
synchronization in some other manner. There is no way of invalidating
a subset of the pages in object cached by fscache, there is no way we
can make O_LAZY work well.

The ceph_readpage_to_fscache() in writepage has no effect and it
should be removed. ceph_readpage_to_fscache() calls cache_valid() to
see if it should perform the page save, and since the file can't have
a CACHE cap at the point in time it doesn't do it.

Thanks,
- Milosz

On Thu, Oct 31, 2013 at 11:56 PM, Li Wang <liwang@...ntukylin.com> wrote:
> Currently, the pages in fscache only are updated in writepage() path,
> add the process in writepages().
>
> Signed-off-by: Min Chen <minchen@...ntukylin.com>
> Signed-off-by: Li Wang <liwang@...ntukylin.com>
> Signed-off-by: Yunchuan Wen <yunchuanwen@...ntukylin.com>
> ---
>  fs/ceph/addr.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 6df8bd4..cc57911 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -746,7 +746,7 @@ retry:
>
>         while (!done && index <= end) {
>                 int num_ops = do_sync ? 2 : 1;
> -               unsigned i;
> +               unsigned i, j;
>                 int first;
>                 pgoff_t next;
>                 int pvec_pages, locked_pages;
> @@ -894,7 +894,6 @@ get_more_pages:
>                 if (!locked_pages)
>                         goto release_pvec_pages;
>                 if (i) {
> -                       int j;
>                         BUG_ON(!locked_pages || first < 0);
>
>                         if (pvec_pages && i == pvec_pages &&
> @@ -924,7 +923,10 @@ get_more_pages:
>
>                 osd_req_op_extent_osd_data_pages(req, 0, pages, len, 0,
>                                                         !!pool, false);
> -
> +               for(j = 0; j < locked_pages; j++) {
> +                       struct page *page = pages[j];
> +                       ceph_readpage_to_fscache(inode, page);
> +               }
>                 pages = NULL;   /* request message now owns the pages array */
>                 pool = NULL;
>
> --
> 1.7.9.5
>
> --
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-- 
Milosz Tanski
CTO
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e: milosz@...in.com
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