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

[Sha Zhengju <handai.szj@gmail.com>]

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'?
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