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From:   Yu Zhao <yuzhao@google.com>
Date:   Tue, 27 Jun 2023 12:33:09 -0600
Message-ID: <CAOUHufa21rX1ODOZ0iJxiy4pstgGqhkbfYozg8-+sRp5ZxAOjA@mail.gmail.com>
Subject: Re: [PATCH v1 10/10] mm: Allocate large folios for anonymous memory
To:     Ryan Roberts <ryan.roberts@arm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Yin Fengwei <fengwei.yin@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-alpha@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
        linux-m68k@lists.linux-m68k.org, linux-s390@vger.kernel.org
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On Tue, Jun 27, 2023 at 3:57=E2=80=AFAM Ryan Roberts <ryan.roberts@arm.com>=
 wrote:
>
> On 27/06/2023 04:01, Yu Zhao wrote:
> > On Mon, Jun 26, 2023 at 11:15=E2=80=AFAM Ryan Roberts <ryan.roberts@arm=
.com> wrote:
> >>
> >> With all of the enabler patches in place, modify the anonymous memory
> >> write allocation path so that it opportunistically attempts to allocat=
e
> >> a large folio up to `max_anon_folio_order()` size (This value is
> >> ultimately configured by the architecture). This reduces the number of
> >> page faults, reduces the size of (e.g. LRU) lists, and generally
> >> improves performance by batching what were per-page operations into
> >> per-(large)-folio operations.
> >>
> >> If CONFIG_LARGE_ANON_FOLIO is not enabled (the default) then
> >> `max_anon_folio_order()` always returns 0, meaning we get the existing
> >> allocation behaviour.
> >>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> >> ---
> >>  mm/memory.c | 159 +++++++++++++++++++++++++++++++++++++++++++++++----=
-
> >>  1 file changed, 144 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index a8f7e2b28d7a..d23c44cc5092 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -3161,6 +3161,90 @@ static inline int max_anon_folio_order(struct v=
m_area_struct *vma)
> >>                 return CONFIG_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX;
> >>  }
> >>
> >> +/*
> >> + * Returns index of first pte that is not none, or nr if all are none=
.
> >> + */
> >> +static inline int check_ptes_none(pte_t *pte, int nr)
> >> +{
> >> +       int i;
> >> +
> >> +       for (i =3D 0; i < nr; i++) {
> >> +               if (!pte_none(ptep_get(pte++)))
> >> +                       return i;
> >> +       }
> >> +
> >> +       return nr;
> >> +}
> >> +
> >> +static int calc_anon_folio_order_alloc(struct vm_fault *vmf, int orde=
r)
> >
> > As suggested previously in 03/10, we can leave this for later.
>
> I disagree. This is the logic that prevents us from accidentally replacin=
g
> already set PTEs, or wandering out of the VMA bounds etc. How would you c=
atch
> all those corener cases without this?

Again, sorry for not being clear previously: we definitely need to
handle alignments & overlapps. But the fallback, i.e., "for (; order >
1; order--) {" in calc_anon_folio_order_alloc() is not necessary.

For now, we just need something like

  bool is_order_suitable() {
    // check whether it fits properly
  }

Later on, we could add

  alloc_anon_folio_best_effort()
  {
    for a list of fallback orders
      is_order_suitable()
  }