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From:   SeongJae Park <sjpark@amazon.com>
To:     Shakeel Butt <shakeelb@google.com>
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Subject: Re: [PATCH v22 05/18] mm/idle_page_tracking: Make PG_(idle|young) reusable
Date:   Thu, 26 Nov 2020 13:31:17 +0100
Message-ID: <20201126123117.23394-1-sjpark@amazon.com>
In-Reply-To: <CALvZod5DGLtegPdDjj72WOO1RmR1MV_8DE+NEakg1PYGurHNUQ@mail.gmail.com>
MIME-Version: 1.0
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On Wed, 25 Nov 2020 07:30:06 -0800 Shakeel Butt <shakeelb@google.com> wrote:

> On Tue, Oct 20, 2020 at 2:04 AM SeongJae Park <sjpark@amazon.com> wrote:
> >
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > PG_idle and PG_young allows the two PTE Accessed bit users,
> > IDLE_PAGE_TRACKING and the reclaim logic concurrently work while don't
> > interfere each other.  That is, when they need to clear the Accessed
> > bit, they set PG_young
> 
> Only PG_young bit

Oops, right.  Maybe I was out of my mind while writing this.  Thank you for
correcting this.

> 
> > and PG_idle to represent the previous state of
> > the bit, respectively.  And when they need to read the bit, if the bit
> > is cleared, they further read the PG_young
> 
> Again only PG_young bit.

Sure.

> 
> PG_idle bit is only read (and set) by the page idle tracking code and
> it can be cleared by others (reclaim or file access).
> 
> > and PG_idle, respectively, to
> > know whether the other has cleared the bit meanwhile or not.
> >
> > We could add another page flag and extend the mechanism to use the flag
> > if we need to add another concurrent PTE Accessed bit user subsystem.
> > However, it would be only waste the space.  Instead, if the new
> > subsystem is mutually exclusive with IDLE_PAGE_TRACKING, it could simply
> > reuse the PG_idle flag.  However, it's impossible because the flags are
> > dependent on IDLE_PAGE_TRACKING.
> >
> > To allow such reuse of the flags, this commit separates the PG_young and
> > PG_idle flag logic from IDLE_PAGE_TRACKING and introduces new kernel
> > config, 'PAGE_IDLE_FLAG'.  Hence, if !IDLE_PAGE_TRACKING and
> > IDLE_PAGE_FLAG, a new subsystem would be able to reuse PG_idle.
> >
> > In the next commit, DAMON's reference implementation of the virtual
> > memory address space monitoring primitives will use it.
> >
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >  include/linux/page-flags.h     |  4 ++--
> >  include/linux/page_ext.h       |  2 +-
> >  include/linux/page_idle.h      |  6 +++---
> >  include/trace/events/mmflags.h |  2 +-
> >  mm/Kconfig                     |  8 ++++++++
> >  mm/page_ext.c                  | 12 +++++++++++-
> >  mm/page_idle.c                 | 10 ----------
> >  7 files changed, 26 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 6be1aa559b1e..7736d290bb61 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -132,7 +132,7 @@ enum pageflags {
> >  #ifdef CONFIG_MEMORY_FAILURE
> >         PG_hwpoison,            /* hardware poisoned page. Don't touch */
> >  #endif
> > -#if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
> > +#if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
> >         PG_young,
> >         PG_idle,
> >  #endif
> > @@ -432,7 +432,7 @@ static inline bool set_hwpoison_free_buddy_page(struct page *page)
> >  #define __PG_HWPOISON 0
> >  #endif
> >
> > -#if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
> > +#if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
> >  TESTPAGEFLAG(Young, young, PF_ANY)
> >  SETPAGEFLAG(Young, young, PF_ANY)
> >  TESTCLEARFLAG(Young, young, PF_ANY)
> > diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
> > index cfce186f0c4e..c9cbc9756011 100644
> > --- a/include/linux/page_ext.h
> > +++ b/include/linux/page_ext.h
> > @@ -19,7 +19,7 @@ struct page_ext_operations {
> >  enum page_ext_flags {
> >         PAGE_EXT_OWNER,
> >         PAGE_EXT_OWNER_ALLOCATED,
> > -#if defined(CONFIG_IDLE_PAGE_TRACKING) && !defined(CONFIG_64BIT)
> > +#if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
> >         PAGE_EXT_YOUNG,
> >         PAGE_EXT_IDLE,
> >  #endif
> > diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
> > index 1e894d34bdce..d8a6aecf99cb 100644
> > --- a/include/linux/page_idle.h
> > +++ b/include/linux/page_idle.h
> > @@ -6,7 +6,7 @@
> >  #include <linux/page-flags.h>
> >  #include <linux/page_ext.h>
> >
> > -#ifdef CONFIG_IDLE_PAGE_TRACKING
> > +#ifdef CONFIG_PAGE_IDLE_FLAG
> >
> >  #ifdef CONFIG_64BIT
> >  static inline bool page_is_young(struct page *page)
> > @@ -106,7 +106,7 @@ static inline void clear_page_idle(struct page *page)
> >  }
> >  #endif /* CONFIG_64BIT */
> >
> > -#else /* !CONFIG_IDLE_PAGE_TRACKING */
> > +#else /* !CONFIG_PAGE_IDLE_FLAG */
> >
> >  static inline bool page_is_young(struct page *page)
> >  {
> > @@ -135,6 +135,6 @@ static inline void clear_page_idle(struct page *page)
> >  {
> >  }
> >
> > -#endif /* CONFIG_IDLE_PAGE_TRACKING */
> > +#endif /* CONFIG_PAGE_IDLE_FLAG */
> >
> >  #endif /* _LINUX_MM_PAGE_IDLE_H */
> > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> > index 5fb752034386..4d182c32071b 100644
> > --- a/include/trace/events/mmflags.h
> > +++ b/include/trace/events/mmflags.h
> > @@ -73,7 +73,7 @@
> >  #define IF_HAVE_PG_HWPOISON(flag,string)
> >  #endif
> >
> > -#if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
> > +#if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
> >  #define IF_HAVE_PG_IDLE(flag,string) ,{1UL << flag, string}
> >  #else
> >  #define IF_HAVE_PG_IDLE(flag,string)
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 19fe2251c87a..044317ef9143 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -761,10 +761,18 @@ config DEFERRED_STRUCT_PAGE_INIT
> >           lifetime of the system until these kthreads finish the
> >           initialisation.
> >
> > +config PAGE_IDLE_FLAG
> > +       bool "Add PG_idle and PG_young flags"
> > +       help
> > +         This feature adds PG_idle and PG_young flags in 'struct page'.  PTE
> > +         Accessed bit writers can set the state of the bit in the flags to let
> > +         other PTE Accessed bit readers don't disturbed.
> > +
> >  config IDLE_PAGE_TRACKING
> >         bool "Enable idle page tracking"
> >         depends on SYSFS && MMU
> >         select PAGE_EXTENSION if !64BIT
> > +       select PAGE_IDLE_FLAG
> >         help
> >           This feature allows to estimate the amount of user pages that have
> >           not been touched during a given period of time. This information can
> > diff --git a/mm/page_ext.c b/mm/page_ext.c
> > index a3616f7a0e9e..f9a6ff65ac0a 100644
> > --- a/mm/page_ext.c
> > +++ b/mm/page_ext.c
> > @@ -58,11 +58,21 @@
> >   * can utilize this callback to initialize the state of it correctly.
> >   */
> >
> 
> Is there a need to move the following code in this patch?

After this patchset, someone would turn CONFIG_PAGE_IDLE_FLAG on but
CONFIG_IDLE_PAGE_TRACKING.  In that case, the build will fail because
page_idle.c will not be compiled.  Because below code is used by page_ext.c
only, I think moving into here is ok.

> 
> 
> > +#if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
> > +static bool need_page_idle(void)
> > +{
> > +       return true;
> > +}
> > +struct page_ext_operations page_idle_ops = {
> > +       .need = need_page_idle,
> > +};
> > +#endif
> > +
> >  static struct page_ext_operations *page_ext_ops[] = {
> >  #ifdef CONFIG_PAGE_OWNER
> >         &page_owner_ops,
> >  #endif
> > -#if defined(CONFIG_IDLE_PAGE_TRACKING) && !defined(CONFIG_64BIT)
> > +#if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
> >         &page_idle_ops,
> >  #endif
> >  };
> > diff --git a/mm/page_idle.c b/mm/page_idle.c
> > index 057c61df12db..144fb4ed961d 100644
> > --- a/mm/page_idle.c
> > +++ b/mm/page_idle.c
> > @@ -211,16 +211,6 @@ static const struct attribute_group page_idle_attr_group = {
> >         .name = "page_idle",
> >  };
> >
> > -#ifndef CONFIG_64BIT
> > -static bool need_page_idle(void)
> > -{
> > -       return true;
> > -}
> > -struct page_ext_operations page_idle_ops = {
> > -       .need = need_page_idle,
> > -};
> > -#endif
> > -
> >  static int __init page_idle_init(void)
> >  {
> >         int err;
> > --
> > 2.17.1
> >
> 
> Overall this patch looks good to me.

Appreciate!


Thanks,
SeongJae Park