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

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Date:   Tue, 27 Nov 2018 16:49:47 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
cc:     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>, Michal Hocko <mhocko@...e.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 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.

In order to do anything with the page struct it would have to obtain a
refcount and possibly lock the page. That would already wait for the
page to become unlocked.