Re: [PATCH V5 2/8] fs/ceph: vfs __set_page_dirty_nobuffers interface instead of doing it inside filesystem

From: "Yan, Zheng" <ukernel@gmail.com>
To: Sha Zhengju <handai.szj@gmail.com>
Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	ceph-devel <ceph-devel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	cgroups@vger.kernel.org, Sage Weil <sage@inktank.com>,
	mhocko@suse.cz, kamezawa.hiroyu@jp.fujitsu.com,
	glommer@gmail.com, Greg Thelen <gthelen@google.com>,
	Wu Fengguang <fengguang.wu@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sha Zhengju <handai.szj@taobao.com>
Subject: Re: [PATCH V5 2/8] fs/ceph: vfs __set_page_dirty_nobuffers interface instead of doing it inside filesystem
Date: Thu, 1 Aug 2013 23:19:48 +0800	[thread overview]
Message-ID: <CAAM7YAmxmmA6g2WPVtGN1-42rtDBYzLhF-gvNXxcBN6dUveBYQ@mail.gmail.com> (raw)
In-Reply-To: <1375357892-10188-1-git-send-email-handai.szj@taobao.com>

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On Thu, Aug 1, 2013 at 7:51 PM, Sha Zhengju <handai.szj@gmail.com> wrote:
> From: Sha Zhengju <handai.szj@taobao.com>
>
> Following we will begin to add memcg dirty page accounting around
__set_page_dirty_
> {buffers,nobuffers} in vfs layer, so we'd better use vfs interface to
avoid exporting
> those details to filesystems.
>
> Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
> ---
>  fs/ceph/addr.c |   13 +------------
>  1 file changed, 1 insertion(+), 12 deletions(-)
>
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 3e68ac1..1445bf1 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -76,7 +76,7 @@ static int ceph_set_page_dirty(struct page *page)
>         if (unlikely(!mapping))
>                 return !TestSetPageDirty(page);
>
> -       if (TestSetPageDirty(page)) {
> +       if (!__set_page_dirty_nobuffers(page)) {

it's too early to set the radix tree tag here. We should set page's
snapshot context and increase the i_wrbuffer_ref first. This is because
once the tag is set, writeback thread can find and start flushing the page.

>                 dout("%p set_page_dirty %p idx %lu -- already dirty\n",
>                      mapping->host, page, page->index);
>                 return 0;
> @@ -107,14 +107,7 @@ static int ceph_set_page_dirty(struct page *page)
>              snapc, snapc->seq, snapc->num_snaps);
>         spin_unlock(&ci->i_ceph_lock);
>
> -       /* now adjust page */
> -       spin_lock_irq(&mapping->tree_lock);
>         if (page->mapping) {    /* Race with truncate? */
> -               WARN_ON_ONCE(!PageUptodate(page));
> -               account_page_dirtied(page, page->mapping);
> -               radix_tree_tag_set(&mapping->page_tree,
> -                               page_index(page), PAGECACHE_TAG_DIRTY);
> -

this code was coped from __set_page_dirty_nobuffers(). I think the reason
Sage did this is to handle the race described in
__set_page_dirty_nobuffers()'s comment. But I'm wonder if "page->mapping ==
NULL" can still happen here. Because truncate_inode_page() unmap page from
processes's address spaces first, then delete page from page cache.

Regards
Yan, Zheng

>                 /*
>                  * Reference snap context in page->private.  Also set
>                  * PagePrivate so that we get invalidatepage callback.
> @@ -126,14 +119,10 @@ static int ceph_set_page_dirty(struct page *page)
>                 undo = 1;
>         }
>
> -       spin_unlock_irq(&mapping->tree_lock);

> -
>         if (undo)
>                 /* whoops, we failed to dirty the page */
>                 ceph_put_wrbuffer_cap_refs(ci, 1, snapc);
>
> -       __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> -
>         BUG_ON(!PageDirty(page));
>         return 1;
>  }
> --
> 1.7.9.5
>
> --
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