On Mon, Dec 20, 2010 at 1:35 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Mon, 20 Dec 2010 11:58:50 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>
>> On Mon, Dec 20, 2010 at 11:27 AM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
>> > On Mon, 20 Dec 2010 11:21:52 +0900 (JST)
>> > 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
>> >> 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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