linux-kernel - Re: [RFC 5/5] truncate: Remove unnecessary page release

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Message-ID: <AANLkTikn-ZyBKwVaDwuQ=QSvB=wLwY40u8FHGyWccStm@mail.gmail.com>
Date:	Mon, 20 Dec 2010 17:09:11 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC 5/5] truncate: Remove unnecessary page release

On Mon, Dec 20, 2010 at 1:35 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Mon, 20 Dec 2010 11:58:50 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
>
>> On Mon, Dec 20, 2010 at 11:27 AM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@...fujitsu.com> wrote:
>> > On Mon, 20 Dec 2010 11:21:52 +0900 (JST)
>> > KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
>> >
>> >> > This patch series changes remove_from_page_cache's page ref counting
>> >> > rule. page cache ref count is decreased in remove_from_page_cache.
>> >> > So we don't need call again in caller context.
>> >> >
>> >> > Cc: Nick Piggin <npiggin@...e.de>
>> >> > Cc: Al Viro <viro@...iv.linux.org.uk>
>> >> > Cc: linux-mm@...ck.org
>> >> > Signed-off-by: Minchan Kim <minchan.kim@...il.com>
>> >> > ---
>> >> >  mm/truncate.c |    1 -
>> >> >  1 files changed, 0 insertions(+), 1 deletions(-)
>> >> >
>> >> > diff --git a/mm/truncate.c b/mm/truncate.c
>> >> > index 9ee5673..8decb93 100644
>> >> > --- a/mm/truncate.c
>> >> > +++ b/mm/truncate.c
>> >> > @@ -114,7 +114,6 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
>> >> >      * calls cleancache_put_page (and note page->mapping is now NULL)
>> >> >      */
>> >> >     cleancache_flush_page(mapping, page);
>> >> > -   page_cache_release(page);       /* pagecache ref */
>> >> >     return 0;
>> >>
>> >> Do we _always_ have stable page reference here? IOW, I can assume
>> >> cleancache_flush_page() doesn't cause NULL deref?
>> >>
>> > Hmm, my review was bad.
>> >
>> > I think cleancache_flush_page() here should eat (mapping, index) as argument
>> > rather than "page".
>> >
>> > BTW,  I can't understand
>> > ==
>> > void __cleancache_flush_page(struct address_space *mapping, struct page *page)
>> > {
>> >        /* careful... page->mapping is NULL sometimes when this is called */
>> >        int pool_id = mapping->host->i_sb->cleancache_poolid;
>> >        struct cleancache_filekey key = { .u.key = { 0 } };
>> > ==
>> >
>> > Why above is safe...
>> > I think (mapping,index) should be passed instead of page.
>>
>> I don't think current code isn't safe.
>>
>> void __cleancache_flush_page(struct address_space *mapping, struct page *page)
>> {
>>         /* careful... page->mapping is NULL sometimes when this is called */
>>         int pool_id = mapping->host->i_sb->cleancache_poolid;
>>         struct cleancache_filekey key = { .u.key = { 0 } };
>>
>>         if (pool_id >= 0) {
>>                 VM_BUG_ON(!PageLocked(page));
>>
>> it does check PageLocked. So caller should hold a page reference to
>> prevent freeing ramined PG_locked
>> If the caller doesn't hold a ref of page, I think it's BUG of caller.
>>
>> In our case, caller calls truncate_complete_page have to make sure it, I think.
>>
>
> Ah, my point is that this function trust page->index even if page->mapping is
> reset to NULL. And I'm not sure that there are any race that an other thread
> add a replacement page for (mapping, index) while a thread call this function.

Because the page is locked and is detached from page cache, I guess
it's no problem.
Anyway, I think It's off-topic.

>
> Thanks,
> -Kame
>
>
>
>



-- 
Kind regards,
Minchan Kim
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