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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Mon, Dec 20, 2010 at 11:32 AM, KOSAKI Motohiro
> <kosaki.motohiro@jp.fujitsu.com> wrote:
> >> On Mon, Dec 20, 2010 at 11:21 AM, KOSAKI Motohiro
> >> <kosaki.motohiro@jp.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@suse.de>
> >> >> Cc: Al Viro <viro@zeniv.linux.org.uk>
> >> >> Cc: linux-mm@kvack.org
> >> >> Signed-off-by: Minchan Kim <minchan.kim@gmail.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
> >>
> >> 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);
}