From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-57.freemail.mail.aliyun.com (out30-57.freemail.mail.aliyun.com [115.124.30.57]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B948D7B for ; Thu, 18 Aug 2022 05:45:39 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R821e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045168;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0VMZQY1e_1660801534; Received: from 30.97.48.48(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0VMZQY1e_1660801534) by smtp.aliyun-inc.com; Thu, 18 Aug 2022 13:45:35 +0800 Message-ID: <098c1480-416d-bca9-cedb-ca495df69b64@linux.alibaba.com> Date: Thu, 18 Aug 2022 13:45:46 +0800 Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH] mm/damon: Validate if the pmd entry is present before accessing To: Muchun Song Cc: sj@kernel.org, Andrew Morton , damon@lists.linux.dev, Linux MM , linux-kernel@vger.kernel.org, Mike Kravetz References: <2838b6737bc259cf575ff11fd1c4b7fdb340fa73.1660717122.git.baolin.wang@linux.alibaba.com> <5173E308-2403-4667-9162-865CF7F11838@linux.dev> <6F99AE03-3C74-4E5B-87E1-6F3232BEEBB5@linux.dev> <8C1A5089-2830-407D-8FF3-2F0F92984645@linux.dev> From: Baolin Wang In-Reply-To: <8C1A5089-2830-407D-8FF3-2F0F92984645@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 8/18/2022 1:12 PM, Muchun Song wrote: > > >> On Aug 18, 2022, at 13:07, Baolin Wang wrote: >> >> >> >> On 8/18/2022 11:39 AM, Muchun Song wrote: >>>> On Aug 18, 2022, at 10:57, Baolin Wang wrote: >>>> >>>> >>>> >>>> 在 8/18/2022 10:41 AM, Muchun Song 写道: >>>>>> On Aug 17, 2022, at 14:21, Baolin Wang wrote: >>>>>> >>>>>> The pmd_huge() is used to validate if the pmd entry is mapped by a huge >>>>>> page, also including the case of non-present (migration or hwpoisoned) >>>>>> pmd entry on arm64 or x86 architectures. Thus we should validate if it >>>>>> is present before making the pmd entry old or getting young state, >>>>>> otherwise we can not get the correct corresponding page. >>>>>> >>>>>> Signed-off-by: Baolin Wang >>>>>> --- >>>>>> mm/damon/vaddr.c | 10 ++++++++++ >>>>>> 1 file changed, 10 insertions(+) >>>>>> >>>>>> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c >>>>>> index 3c7b9d6..1d16c6c 100644 >>>>>> --- a/mm/damon/vaddr.c >>>>>> +++ b/mm/damon/vaddr.c >>>>>> @@ -304,6 +304,11 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr, >>>>>> >>>>>> if (pmd_huge(*pmd)) { >>>>>> ptl = pmd_lock(walk->mm, pmd); >>>>>> + if (!pmd_present(*pmd)) { >>>>> Unluckily, we should use pte_present here. See commit c9d398fa23788. We can use >>>>> huge_ptep_get() to get a hugetlb pte, so it’s better to put the check after >>>>> pmd_huge. >>>> >>>> IMO this is not the case for hugetlb, and the hugetlb case will be handled by damon_mkold_hugetlb_entry(), which already used pte_present() for hugetlb case. >>> Well, I thought it is hugetlb related since I saw the usage of pmd_huge. If it is THP case, why >>> not use pmd_trans_huge? >> >> IIUC, it can not guarantee the pmd is present if pmd_trans_huge() returns true on all architectures, at least on X86, we still need pmd_present() validation. So changing to pmd_trans_huge() does not make code simpler from my side, and I prefer to keep this patch. > > I am not suggesting you change it to pmd_trans_huge() in this patch, I am just expressing > my curious. At least, it is a little confusing to me. OK. >> >> Maybe we can send another cleanup patch to replace pmd_huge() with pmd_trans_huge() for THP case to make code more readable? How do you think? Thanks. > > Yep, make sense to me. OK. I can add a cleanup patch in next version. Thanks for your input.