linux-kernel - Re: [PATCH 01/10] mm: page_ref_add_unless() does not trace 'u' argument

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Message-ID: <CA+CK2bANo3VJ=8VouqqnE8QWUMg30_Sospizk6eXozpzgUB7VA@mail.gmail.com>
Date:   Thu, 9 Dec 2021 10:15:59 -0500
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        linux-m68k@...ts.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        william.kucharski@...cle.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        schmitzmic@...il.com, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>,
        Muchun Song <songmuchun@...edance.com>, weixugc@...gle.com,
        Greg Thelen <gthelen@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 01/10] mm: page_ref_add_unless() does not trace 'u' argument

On Wed, Dec 8, 2021 at 9:17 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Wed, Dec 08, 2021 at 08:25:22PM -0500, Pasha Tatashin wrote:
> > On Wed, Dec 8, 2021 at 3:55 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Wed, Dec 08, 2021 at 08:35:35PM +0000, Pasha Tatashin wrote:
> > > > In other page_ref_* functions all arguments and returns are traced, but
> > > > in page_ref_add_unless the 'u' argument which stands for unless boolean
> > > > is not traced. However, what is more confusing is that in the tracing
> > > > routine:
> > > >       __page_ref_mod_unless(struct page *page, int v, int u);
> > > >
> > > > The 'u' argument present, but instead a return value is passed into
> > > > this argument.
> > > >
> > > > Add a new template specific for page_ref_add_unless(), and trace all
> > > > arguments and the return value.
> > >
> > > The special casing of '1' for device pages is going away, so NAK
> > > to this user-visible change.
> >
> > I can drop this patch, as it really intended to fix existing oddities
> > and missing info. However, I do not really understand your NAK reason.
> > Can you please explain about the special casing of "1" for device
> > pages?
>
> $ git grep page_ref_add_unless
> include/linux/mm.h:     return page_ref_add_unless(page, 1, 0);
> include/linux/page_ref.h:static inline bool page_ref_add_unless(struct page *page, int nr, int u)
> include/linux/page_ref.h:       return page_ref_add_unless(&folio->page, nr, u);
> mm/memcontrol.c:                if (!page_ref_add_unless(page, 1, 1))
>
> 'u' is always 0, except for the caller in mm/memcontrol.c:
>
>         if (is_device_private_entry(ent)) {
>                 page = pfn_swap_entry_to_page(ent);
>                 /*
>                  * MEMORY_DEVICE_PRIVATE means ZONE_DEVICE page and which have
>                  * a refcount of 1 when free (unlike normal page)
>                  */
>                 if (!page_ref_add_unless(page, 1, 1))
>                         return NULL;
>                 return page;
>         }
>
> That special casing of ZONE_DEVICE pages is being fixed, so 'u' will
> soon always be 0, and I'm sure we'll delete it as an argument.  So
> there's no point in tracing what it 'used to be'.

Sounds good, at the time when 'u' is deleted we can also address
inconsistencies fixed in this patch. So, I will drop it from my series
in the next version.

Thanks,
Pasha