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s=arc-20220608; d=hostedemail.com; t=1691584643; a=rsa-sha256; cv=none; b=vH31Vuek0NeJZy+aHYTN/U81JBACa2XxqplMXdCRIZVBf1gON2KsqcdpZ6uaipWRs1+HMU VKGV4nImNhMRnPS2W0nmsmwLGmhQ+pWTTw7rGW276t1PjnZXjgq31JE/Q6MTZSD+ReVACS lGDV4F/GfZ5M8SyxDB1yq28HC9bAQHk= ARC-Authentication-Results: i=1; imf27.hostedemail.com; dkim=none; spf=pass (imf27.hostedemail.com: domain of wangkefeng.wang@huawei.com designates 45.249.212.187 as permitted sender) smtp.mailfrom=wangkefeng.wang@huawei.com; dmarc=pass (policy=quarantine) header.from=huawei.com Received: from dggpemm100001.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4RLV0N4qggzmXSV; Wed, 9 Aug 2023 20:36:04 +0800 (CST) Received: from [10.174.177.243] (10.174.177.243) by dggpemm100001.china.huawei.com (7.185.36.93) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.27; Wed, 9 Aug 2023 20:37:16 +0800 Message-ID: Date: Wed, 9 Aug 2023 20:37:15 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration() Content-Language: en-US To: Zi Yan , Mike Kravetz CC: Naoya Horiguchi , Matthew Wilcox , Andrew Morton , , , Huang Ying , David Hildenbrand , Mike Kravetz References: <20230802095346.87449-1-wangkefeng.wang@huawei.com> <20230802095346.87449-2-wangkefeng.wang@huawei.com> <001ee9b0-ea25-a896-e3ae-9a9b05a46546@huawei.com> <5BBFF5D3-3416-4C0E-9FDD-655661657D67@nvidia.com> From: Kefeng Wang In-Reply-To: <5BBFF5D3-3416-4C0E-9FDD-655661657D67@nvidia.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemm100001.china.huawei.com (7.185.36.93) X-CFilter-Loop: Reflected X-Rspamd-Queue-Id: BCFDC40008 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: njb4jh4q3cbf4a9jeihicb4j3pkg5z3y X-HE-Tag: 1691584641-553206 X-HE-Meta: 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 wuRDJy8P s0FTCE9blR/G8azdW5Sr+82IEMoREe/HBBpUMigrjr3eRoGDvv8iN/ToYy8eEWaCvukZg2FqEruvDQTG8ooTfEEk/Y46rkXP9mrokUkrmiR8qx/9UTxzr/+2WxJgMZo8ELuiimGrDFpsInpazeC+tzNuBq0td6VDassapX9H17TgUNYpKXReC5a1bimw0wvPdxh1KSFtOxNJJqppP6Ntwr9HzwsHqBj+HdryfiVXvOPhpbs+EhaM8VfVViQpBfY6C/9sTSVVNsSXULAjhlUpw+5CKIVHpvwaFYcnvqiZ+rKYNWy6+5BnLQXsJLw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hi Mike On 2023/8/8 2:45, Zi Yan wrote: > On 7 Aug 2023, at 8:20, Kefeng Wang wrote: > >> Hi Zi Yan and Matthew and Naoya, >> >> On 2023/8/4 13:54, Kefeng Wang wrote: >>> >>> >>> On 2023/8/4 10:42, Zi Yan wrote: >>>> On 3 Aug 2023, at 21:45, Kefeng Wang wrote: >>>> >>>>> On 2023/8/3 20:30, Matthew Wilcox wrote: >>>>>> On Thu, Aug 03, 2023 at 03:13:21PM +0800, Kefeng Wang wrote: >>>>>>> >> >> ... >> >>>>> >>>>> >>>>>    if (PageHuge(page))  // page must be a hugetlb page >>>>>     if (PageHead(page)) // page must be a head page, not tail >>>>>               isolate_hugetlb() // isolate the hugetlb page if head >>>>> >>>>> After using folio, >>>>> >>>>>    if (folio_test_hugetlb(folio)) // only check folio is hugetlb or not >>>>> >>>>> I don't check the page is head or not, since the follow_page could >>>>> return a sub-page, so the check PageHead need be retained, right? >>>> >>>> Right. It will prevent the kernel from trying to isolate the same hugetlb page >>>> twice when two pages are in the same hugetlb folio. But looking at the >>>> code, if you try to isolate an already-isolated hugetlb folio, isolate_hugetlb() >>>> would return false, no error would show up. But it changes err value >>>> from -EACCES to -EBUSY and user will see a different page status than before. >>> >> >> Before e66f17ff7177 ("mm/hugetlb: take page table lock in follow_huge_pmd()") >> in v4.0, follow_page() will return NULL on tail page for Huagetlb page, >> and move_pages() will return -ENOENT errno,but after that commit, >> -EACCES is returned, which not match the manual, >> >>> >>> When check man[1], the current -EACCES is not right, -EBUSY is not >>> precise but more suitable for this scenario, >>> >>>      -EACCES >>>               The page is mapped by multiple processes and can be moved >>>               only if MPOL_MF_MOVE_ALL is specified. >>> >>>      -EBUSY The page is currently busy and cannot be moved.  Try again >>>               later.  This occurs if a page is undergoing I/O or another >>>               kernel subsystem is holding a reference to the page. >>>     -ENOENT >>>               The page is not present. >>> >>>> >>>> I wonder why we do not have follow_folio() and returns -ENOENT error pointer >>>> when addr points to a non head page. It would make this patch more folio if >>>> follow_folio() can be used in place of follow_page(). One caveat is that >>>> user will see -ENOENT instead of -EACCES after this change. >>>> >>> >>> -ENOENT is ok, but maybe the man need to be updated too. >> >> According to above analysis, -ENOENT is suitable when introduce the >> follow_folio(), but when THP migrate support is introduced by >> e8db67eb0ded ("mm: migrate: move_pages() supports thp migration") in >> v4.14, the tail page will be turned into head page and return -EBUSY, >> >> So should we unify errno(maybe use -ENOENT) about the tail page? >> >> >>> >>> >>> >>> [1] https://man7.org/linux/man-pages/man2/move_pages.2.html > > I think so. I think -EBUSY is more reasonable for tail pages. But there is > some subtle difference between THP and hugetlb from current code: > > For THP, compound_head() is used to get the head page for isolation, this means > if user specifies a tail page address in move_pages(), the whole THP can be > migrated. > > For hugetlb, only if user specifies the head page address of a hugetlb page, > the hugetlb page will be migrated. Otherwise, an error would show up. > > Cc Mike to help us clarify the expected behavior of hugetlb. > > Hi Mike, what is the expected behavior, if a user tries to use move_pages() > to migrate a non head page of a hugetlb page? Could you give some advise, thanks > > -- > Best Regards, > Yan, Zi