Messages in this thread | | | Date | Thu, 18 Aug 2022 10:39:54 +0800 | Subject | Re: [PATCH] mm/damon: Validate if the pmd entry is present before accessing | From | Baolin Wang <> |
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On 8/18/2022 10:29 AM, SeongJae Park wrote: > Hi Baolin, > > On Thu, 18 Aug 2022 09:05:58 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote: > >> >> >> On 8/18/2022 12:09 AM, SeongJae Park wrote: >>> Hi Baolin, >>> >>> >>> Thank you always for your great patch! >>> >>> On Wed, 17 Aug 2022 14:21:12 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> 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. >>> >>> Maybe I'm missing something, but... I'm unsure if the page is present or not >>> really matters from the perspective of access checking. In the case, DAMON >>> could simply report the page has accessed once for the first check after the >>> page being non-present if it really accessed before, and then report the page >>> as not accessed, which is true. >> >> Yes, that's the patch's goal to make the accesses correct. However if >> the PMD entry is not present, we can not get the correct page object by >> pmd_pfn(*pmd), since the non-present pmd entry will contain swap type >> and swap offset with below format on ARM64, that means the pfn number is >> saved in bits 8-57 in a migration or poisoned entry, but pmd_pfn() still >> treat bits 12-47 as the pfn number on ARM64, which may get an incorrect >> page struct (also maybe is NULL by pfn_to_online_page()) to make the >> access statistics incorrect. >> >> /* >> * Encode and decode a swap entry: >> * bits 0-1: present (must be zero) >> * bits 2: remember PG_anon_exclusive >> * bits 3-7: swap type >> * bits 8-57: swap offset >> * bit 58: PTE_PROT_NONE (must be zero) >> */ >> >> >> Moreoever I don't think we should still waste time to get the page of >> the non-present entry, just treat it as not-accessed and skip it, that >> keeps consistent with non-present pte level entry. >> >> Does that make sense for you? Thanks. > > Yes, that totally makes sense. Thank you very much for the kind answer. I > think it would be great if we could put the detailed explanation in the commit > message. Could you please update the commit message and post v2 of the patch?
Sure, will update the commit message to make it more clear and I think that can also answer Andrew's concern.
> > Reviewed-by: SeongJae Park <sj@kernel.org>
Thanks.
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