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References: <20221222061341.381903-1-yuanchu@google.com> <20221222104937.795d2a134ac59c8244d9912c@linux-foundation.org>
In-Reply-To: <20221222104937.795d2a134ac59c8244d9912c@linux-foundation.org>
From:   Yu Zhao <yuzhao@google.com>
Date:   Thu, 22 Dec 2022 12:44:35 -0700
Message-ID: <CAOUHufZpbfTCeqteEZt5k-kFZh3-++Gm0Wnc0-O=RFT-K9kzkw@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm: add vma_has_locality()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Yuanchu Xie <yuanchu@google.com>,
        Ivan Babrou <ivan@cloudflare.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Steven Barrett <steven@liquorix.net>,
        Brian Geffon <bgeffon@google.com>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Suren Baghdasaryan <surenb@google.com>,
        Arnd Bergmann <arnd@arndb.de>, Peter Xu <peterx@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Gaosheng Cui <cuigaosheng1@huawei.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
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On Thu, Dec 22, 2022 at 11:49 AM Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> On Wed, 21 Dec 2022 22:13:40 -0800 Yuanchu Xie <yuanchu@google.com> wrote:
>
> > From: Yu Zhao <yuzhao@google.com>

This works; suggested-by probably works even better, since I didn't do
the follow-up work.

> > Currently in vm_flags in vm_area_struct, both VM_SEQ_READ and
> > VM_RAND_READ indicate a lack of locality in accesses to the vma. Some
> > places that check for locality are missing one of them. We add
> > vma_has_locality to replace the existing locality checks for clarity.
>
> I'm all confused.  Surely VM_SEQ_READ implies locality and VM_RAND_READ
> indicates no-locality?

Spatially, yes. But we focus more on the temporal criteria here, i.e.,
the reuse of an area within a relatively small duration. Both the
active/inactive LRU and MGLRU rely on this.

VM_SEQ_READ, while being a special case of spatial locality, fails the
temporal criteria. VM_RAND_READ fails both criterias, obviously.

Once an area passes the temporal criteria, MGLRU additionally exploits
spatial locality by lru_gen_look_around(), which is also touched in
this patch. This part is good to know but not really relevant here.