linux-kernel - Re: [PATCH v3] mm/swap: Fix release_pages() when releasing devmap pages

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Date:   Tue, 4 Jun 2019 13:32:47 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH v3] mm/swap: Fix release_pages() when releasing devmap
 pages

On Tue, Jun 04, 2019 at 01:17:42PM -0700, John Hubbard wrote:
> On 6/4/19 1:11 PM, Dan Williams wrote:
> > On Tue, Jun 4, 2019 at 12:48 PM John Hubbard <jhubbard@...dia.com> wrote:
> >>
> >> On 6/4/19 9:48 AM, ira.weiny@...el.com wrote:
> >>> From: Ira Weiny <ira.weiny@...el.com>
> >>>
> ...
> >>> diff --git a/mm/swap.c b/mm/swap.c
> >>> index 7ede3eddc12a..6d153ce4cb8c 100644
> >>> --- a/mm/swap.c
> >>> +++ b/mm/swap.c
> >>> @@ -740,15 +740,20 @@ void release_pages(struct page **pages, int nr)
> >>>               if (is_huge_zero_page(page))
> >>>                       continue;
> >>>
> >>> -             /* Device public page can not be huge page */
> >>> -             if (is_device_public_page(page)) {
> >>> +             if (is_zone_device_page(page)) {
> >>>                       if (locked_pgdat) {
> >>>                               spin_unlock_irqrestore(&locked_pgdat->lru_lock,
> >>>                                                      flags);
> >>>                               locked_pgdat = NULL;
> >>>                       }
> >>> -                     put_devmap_managed_page(page);
> >>> -                     continue;
> >>> +                     /*
> >>> +                      * Not all zone-device-pages require special
> >>> +                      * processing.  Those pages return 'false' from
> >>> +                      * put_devmap_managed_page() expecting a call to
> >>> +                      * put_page_testzero()
> >>> +                      */
> >>
> >> Just a documentation tweak: how about:
> >>
> >>                         /*
> >>                          * ZONE_DEVICE pages that return 'false' from
> >>                          * put_devmap_managed_page() do not require special
> >>                          * processing, and instead, expect a call to
> >>                          * put_page_testzero().
> >>                          */
> > 
> > Looks better to me, but maybe just go ahead and list those
> > expectations explicitly. Something like:
> > 
> >                         /*
> >                          * put_devmap_managed_page() only handles
> >                          * ZONE_DEVICE (struct dev_pagemap managed)
> >                          * pages when the hosting dev_pagemap has the
> >                          * ->free() or ->fault() callback handlers
> >                          *  implemented as indicated by
> >                          *  dev_pagemap.type. Otherwise the expectation
> >                          *  is to fall back to a plain decrement /
> >                          *  put_page_testzero().
> >                          */
> 
> I like it--but not here, because it's too much internal detail in a
> call site that doesn't use that level of detail. The call site looks
> at the return value, only.
> 
> Let's instead put that blurb above (or in) the put_devmap_managed_page() 
> routine itself. And leave the blurb that I wrote where it is. And then I
> think everything will have an appropriate level of detail in the right places.

I agree.  This leaves it open that this handles any special processing which is
required.

FWIW the same call is made in put_page() and has no comment so perhaps we are
getting wrapped around the axle for no reason?

Frankly I questioned myself when I mentioned put_page_testzero() as well.  But
I'm ok with Johns suggestion.  My wording was a bit "rushed".  Sorry about
that.  I wanted to remove the word 'fail' from the comment because I think it
is what caught Michal's eye.

Ira

> 
> 
> thanks,
> -- 
> John Hubbard
> NVIDIA
>