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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 4 Jan 2022 13:34:18 -0800
Message-ID: <CAHk-=wib5-tUrf2=zYL9hjCqqFykZmTr_-vMAvSo48boCA+-Wg@mail.gmail.com>
Subject: Re: [PATCH v6 4/9] mm: multigenerational lru: groundwork
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Jesse Barnes <jsbarnes@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        Ying Huang <ying.huang@intel.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, page-reclaim@google.com,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Konstantin Kharlamov <Hi-Angel@yandex.ru>
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On Tue, Jan 4, 2022 at 12:23 PM Yu Zhao <yuzhao@google.com> wrote:
>

> index a7e4a9e7d807..fadbf8e6abcd 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
>
> +#ifdef CONFIG_LRU_GEN
> +static inline void task_enter_lru_fault(void)
> +{
> +       WARN_ON_ONCE(current->in_lru_fault);
...

Why are these in this very core header file?

They are used in one single file - mm/memory.c.

They should be just static functions there.

I'm also not sure why the calling convention is

        if (lru_fault)
                task_enter_lru_fault();

instead of doing just

        task_enter_lru_fault(vma);

and having that function do

        /* Don't do LRU fault accounting for SEQ/RAND files */
        if (unlikely(vma->vm_flags & (VM_SEQ_READ | VM_RAND_READ)))
                return;

which would seem to be a lot more legible and straightforward.

In fact, you could do it without any conditionals at all, if you just
remove the WARN_ON_ONCE() from the exit path, turning it into just

        current->in_lru_fault = !(vma->vm_flags & (VM_SEQ_READ | VM_RAND_READ));

for 'enter' and just

        current->in_lru_fault = 0;

for exit.

It seems pointless to have that extra variable, and the extra
conditionals, for a case that is probably very unusual indeed.

                  Linus