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

[Minchan Kim]

On Mon, Dec 20, 2010 at 12:03 PM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>> On Mon, Dec 20, 2010 at 11:32 AM, KOSAKI Motohiro
>> <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>> >> On Mon, Dec 20, 2010 at 11:21 AM, KOSAKI Motohiro
>> >> <kosaki.motohiro@xxxxxxxxxxxxxx> 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@xxxxxxx>
>> >> >> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> >> >> Cc: linux-mm@xxxxxxxxx
>> >> >> Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
>> >> >> ---
>> >> >>  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
>> >>
>> >> I think so.
>> >> Because the page is locked so caller have to hold a ref to unlock it.
>> >
>> > Hmm...
>> >
>> > Perhaps, I'm missing something. But I think  __memory_failure() only lock
>> > compaund_head page. not all. example.
>>
>> The page passed truncate_complete_page is only head page?
>> Is it possible to pass the page which isn't head of compound in
>> truncate_complete_page?
>
> I dunno, really. My five miniture grep found following logic. therefore I asked you.
>
>
>
> __memory_failure()
> {
>        p = pfn_to_page(pfn);
>        hpage = compound_head(p);
> (snip)
>        res = -EBUSY;
>        for (ps = error_states;; ps++) {
>                if ((p->flags & ps->mask) == ps->res) {
>                        res = page_action(ps, p, pfn);  // call truncate here
>                        break;
>                }
>        }
> out:
>        unlock_page(hpage);
> }
>
>

AFAIK, We have to handle head page when we handle compound page.
Internal page handling logic about tail pages is hidden by compound
page internal.

So I think memory_failure also don't have a problem.
For needing double check, Cced Andi.

Thanks for the review, KOSAKI.


-- 
Kind regards,
Minchan Kim
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