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    SubjectRe: [RFC 5/5] truncate: Remove unnecessary page release
    On Mon, 20 Dec 2010 11:58:50 +0900
    Minchan Kim <minchan.kim@gmail.com> wrote:

    > On Mon, Dec 20, 2010 at 11:27 AM, KAMEZAWA Hiroyuki
    > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > On Mon, 20 Dec 2010 11:21:52 +0900 (JST)
    > > 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
    > >> 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.

    Thanks,
    -Kame



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