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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.LSU.2.11.1711041713400.5450@eggly.anvils>
Date:   Sat, 4 Nov 2017 17:28:05 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Hugh Dickins <hughd@...gle.com>, 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 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).
Think about it, the mayhem that would follow from a page being freed while
on pagevec: of course it must hold a reference.  The only surprise is that
the extra reference is not needed while on LRU: one can think of PageLRU
as an extension of the page count.

Hugh