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 <cgroups@vger.kernel.org>,
	Sage Weil <sage@inktank.com>, 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 21:11:38 +0800	[thread overview]
Message-ID: <CAAM7YAnVYZ-=LXVuDHj38Bh88Fd+Rw9w7Efs1_efmv9X90-D1Q@mail.gmail.com> (raw)
In-Reply-To: <CAFj3OHXR=PqHwLcXCub8YezcGFF7kbKX3X2dV=6FQM78K__D8g@mail.gmail.com>

On Fri, Aug 2, 2013 at 5:04 PM, Sha Zhengju <handai.szj@gmail.com> wrote:
>
> On Thu, Aug 1, 2013 at 11:19 PM, Yan, Zheng <ukernel@gmail.com> 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.
>
> OK, thanks for pointing it out.
>
> >
> >
> >>                 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.
>
> But in non-mmap case, doesn't it has no relation to 'unmap page from
> address spaces'?

In non-mmap case, page is locked when the set_page_dirty() callback is called.
truncate_inode_page() waits until the page is unlocked, then delete it from the
page cache.

Regards
Yan, Zheng

> The check is exactly avoiding racy with delete_from_page_cache(),
> since the two both need to hold mapping->tree_lock, and if truncate
> goes first then __set_page_dirty_nobuffers() may have NULL mapping.
>
>
> Thanks,
> Sha

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