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

From: Sha Zhengju <handai.szj@gmail.com>
To: Sage Weil <sage@inktank.com>
Cc: "Yan, Zheng" <ukernel@gmail.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	ceph-devel <ceph-devel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>, Cgroups <cgroups@vger.kernel.org>,
	Michal Hocko <mhocko@suse.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
	Glauber Costa <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: Fri, 2 Aug 2013 18:04:17 +0800	[thread overview]
Message-ID: <CAFj3OHVXvtr5BDMrGatHZi7M9y+dh1ZKRMQZGjZmNBcg3pNQtw@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.00.1308011121080.22584@cobra.newdream.net>

On Fri, Aug 2, 2013 at 2:27 AM, Sage Weil <sage@inktank.com> wrote:
> On Thu, 1 Aug 2013, Yan, Zheng wrote:
>> 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.
>
> Unfortunately I only remember being frustrated by this code.  :)  Looking
> at it now, though, it seems like the minimum fix is to set the
> page->private before marking the page dirty.  I don't know the locking
> rules around that, though.  If that is potentially racy, maybe the safest
> thing would be if __set_page_dirty_nobuffers() took a void* to set
> page->private to atomically while holding the tree_lock.
>

Sorry, I don't catch the point of your last sentence... Could you
please explain it again?

I notice there is a check in __set_page_dirty_nobuffers():
      WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
So does it mean we can only set page->private after it? but if so the
__mark_inode_dirty is still ahead of setting snapc.

Thanks,
Sha

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