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

[Minchan Kim]

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.

>
>
> -Kame
>
>



-- 
Kind regards,
Minchan Kim
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