linux-kernel - Re: [PATCH 1/2] shmem: drop lru_add_drain_all from shmem_wait_for

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Date:   Sun, 5 Nov 2017 08:37:32 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     linux-mm@...ck.org, Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Tejun Heo <tj@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Herrmann <dh.herrmann@...il.com>
Subject: Re: [PATCH 1/2] shmem: drop lru_add_drain_all from
 shmem_wait_for_pins

On Sat 04-11-17 17:28:05, Hugh Dickins wrote:
> On Fri, 3 Nov 2017, Michal Hocko wrote:
> > On Fri 03-11-17 00:46:18, Hugh Dickins wrote:
> > > 
> > > NAK.  shmem_wait_for_pins() is waiting for temporary pins on the pages
> > > to go away, and using lru_add_drain_all() in the usual way, to lower
> > > the refcount of pages temporarily pinned in a pagevec somewhere.  Page
> > > count is touched by draining pagevecs: I'm surprised to see you say
> > > that it isn't - or have pagevec page references been eliminated by
> > > a recent commit that I missed?
> > 
> > I must be missing something here. __pagevec_lru_add_fn merely about
> > moving the page into the appropriate LRU list, pagevec_move_tail only
> > rotates, lru_deactivate_file_fn moves from active to inactive LRUs,
> > lru_lazyfree_fn moves from anon to file LRUs and activate_page_drain
> > just moves to the active list. None of those operations touch the page
> > count AFAICS. So I would agree that some pages might be pinned outside
> > of the LRU (lru_add_pvec) and thus unreclaimable but does this really
> > matter. Or what else I am missing?
> 
> Line 213 of mm/swap.c?  Where pagevec_lru_move_fn() calls release_pages()
> to release the extra references (which each page came in with when added).

I am obviously blind. I was staring at that function many times simply
missing this part. My bad and sorry about not taking a deeper look.
Shame...
-- 
Michal Hocko
SUSE Labs