Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.33 as permitted sender) client-ip=23.128.96.33;
Message-ID: <3b56b26b-a550-4e06-b355-55564b40cfb5@huawei.com>
Date:   Wed, 11 Oct 2023 11:02:13 +0800
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH -next 1/7] mm_types: add _last_cpupid into folio
Content-Language: en-US
To:     Matthew Wilcox <willy@infradead.org>
CC:     Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <ying.huang@intel.com>,
        <david@redhat.com>, Zi Yan <ziy@nvidia.com>
References: <20231010064544.4162286-1-wangkefeng.wang@huawei.com>
 <20231010064544.4162286-2-wangkefeng.wang@huawei.com>
 <ZSVEmhPCjZKyp97a@casper.infradead.org>
From:   Kefeng Wang <wangkefeng.wang@huawei.com>
In-Reply-To: <ZSVEmhPCjZKyp97a@casper.infradead.org>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 2023/10/10 20:33, Matthew Wilcox wrote:
> On Tue, Oct 10, 2023 at 02:45:38PM +0800, Kefeng Wang wrote:
>> At present, only arc/sparc/m68k define WANT_PAGE_VIRTUAL, both of
>> them don't support numa balancing, and the page struct is aligned
>> to _struct_page_alignment, it is safe to move _last_cpupid before
>> 'virtual' in page, meanwhile, add it into folio, which make us to
>> use folio->_last_cpupid directly.
> 
> What do you mean by "safe"?  I think you mean "Does not increase the
> size of struct page", but if that is what you mean, why not just say so?
> If there's something else you mean, please explain.

Don't increase size of struct page and don't impact the real order of
struct page as the above three archs without numa balancing support.

> 
> In any event, I'd like to see some reasoning that _last_cpupid is actually
> information which is logically maintained on a per-allocation basis,
> not a per-page basis (I think this is true, but I honestly don't know)

The _last_cpupid is updated in should_numa_migrate_memory() from numa
fault(do_numa_page, and do_huge_pmd_numa_page), it is per-page(normal
page and PMD-mapped page). Maybe I misunderstand your mean, please
correct me.

> 
> And looking at all this, I think it makes sense to move _last_cpupid
> before the kmsan garbage, then add both 'virtual' and '_last_cpupid'
> to folio.

sure, I will add both of them into folio and don't re-order 'virtual' 
and '_last_cpupid'.
> 
>