linux-kernel - Re: [PATCHi v2] mm: put_and_wait_on_page

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Message-ID: <20181127165659.GC6923@dhcp22.suse.cz>
Date:   Tue, 27 Nov 2018 17:56:59 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        David Herrmann <dh.herrmann@...il.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Piggin <npiggin@...il.com>, pifang@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHi v2] mm: put_and_wait_on_page_locked() while page is
 migrated

On Tue 27-11-18 16:49:47, Cristopher Lameter wrote:
> On Tue, 27 Nov 2018, Mike Rapoport wrote:
> 
> > >  * @page: The page to wait for.
> > >  *
> > >  * The caller should hold a reference on @page.  They expect the page to
> > >  * become unlocked relatively soon, but do not wish to hold up migration
> > >  * (for example) by holding the reference while waiting for the page to
> > >  * come unlocked.  After this function returns, the caller should not
> > >  * dereference @page.
> > >  */
> >
> > How about:
> >
> > They expect the page to become unlocked relatively soon, but they can wait
> > for the page to come unlocked without holding the reference, to allow
> > other users of the @page (for example migration) to continue.
> 
> All of this seems a bit strange and it seems unnecessary? Maybe we need a
> better explanation?
> 
> A process has no refcount on a page struct and is waiting for it to become
> unlocked? Why? Should it not simply ignore that page and continue? It
> cannot possibly do anything with the page since it does not hold a
> refcount.

So do you suggest busy waiting on the page under migration?
-- 
Michal Hocko
SUSE Labs