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Date:   Wed, 1 Sep 2021 02:25:16 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     akpm@linux-foundation.org, tglx@linutronix.de, hannes@cmpxchg.org,
        mhocko@kernel.org, vdavydov.dev@gmail.com,
        kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com,
        david@redhat.com, vbabka@suse.cz, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        songmuchun@bytedance.com
Subject: Re: [PATCH v2 2/2] mm: remove redundant smp_wmb()
Message-ID: <20210831232516.4ft2kkn63lngggbb@box.shutemov.name>
References: <20210831132111.85437-1-zhengqi.arch@bytedance.com>
 <20210831132111.85437-3-zhengqi.arch@bytedance.com>
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On Tue, Aug 31, 2021 at 09:21:11PM +0800, Qi Zheng wrote:
> The smp_wmb() which is in the __pte_alloc() is used to
> ensure all ptes setup is visible before the pte is made
> visible to other CPUs by being put into page tables. We
> only need this when the pte is actually populated, so
> move it to pmd_install(). __pte_alloc_kernel(),
> __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar
> to this case.
> 
> We can also defer smp_wmb() to the place where the pmd entry
> is really populated by preallocated pte. There are two kinds
> of user of preallocated pte, one is filemap & finish_fault(),
> another is THP. The former does not need another smp_wmb()
> because the smp_wmb() has been done by pmd_install().
> Fortunately, the latter also does not need another smp_wmb()
> because there is already a smp_wmb() before populating the
> new pte when the THP uses a preallocated pte to split a huge
> pmd.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: David Hildenbrand <david@redhat.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov