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Violators will be prosecuted for from ; Fri, 7 Sep 2018 05:15:18 -0600 Subject: Re: [RFC PATCH V2 4/4] powerpc/mm/iommu: Allow migration of cma allocated pages during mm_iommu_get References: <20180906054342.25094-1-aneesh.kumar@linux.ibm.com> <20180906054342.25094-4-aneesh.kumar@linux.ibm.com> <20180906125356.GX14951@dhcp22.suse.cz> <50d355bf-17d0-ee01-ec35-7f04e79ca277@linux.ibm.com> <20180907090312.GF19621@dhcp22.suse.cz> From: "Aneesh Kumar K.V" Date: Fri, 7 Sep 2018 16:45:09 +0530 MIME-Version: 1.0 In-Reply-To: <20180907090312.GF19621@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: <8337fdcc-3344-04dd-ddb2-68f86912f333@linux.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: akpm@linux-foundation.org, Alexey Kardashevskiy , mpe@ellerman.id.au, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org On 09/07/2018 02:33 PM, Michal Hocko wrote: > On Thu 06-09-18 19:00:43, Aneesh Kumar K.V wrote: >> On 09/06/2018 06:23 PM, Michal Hocko wrote: >>> On Thu 06-09-18 11:13:42, Aneesh Kumar K.V wrote: >>>> Current code doesn't do page migration if the page allocated is a compound page. >>>> With HugeTLB migration support, we can end up allocating hugetlb pages from >>>> CMA region. Also THP pages can be allocated from CMA region. This patch updates >>>> the code to handle compound pages correctly. >>>> >>>> This use the new helper get_user_pages_cma_migrate. It does one get_user_pages >>>> with right count, instead of doing one get_user_pages per page. That avoids >>>> reading page table multiple times. >>>> >>>> The patch also convert the hpas member of mm_iommu_table_group_mem_t to a union. >>>> We use the same storage location to store pointers to struct page. We cannot >>>> update alll the code path use struct page *, because we access hpas in real mode >>>> and we can't do that struct page * to pfn conversion in real mode. >>> >>> I am not fmailiar with this code so bear with me. I am completely >>> missing the purpose of this patch. The changelog doesn't really explain >>> that AFAICS. I can only guess that you do not want to establish long >>> pins on CMA pages, right? So whenever you are about to pin a page that >>> is in CMA you migrate it away to a different !__GFP_MOVABLE page, right? >> >> That is right. >> >>> If that is the case then how do you handle pins which are already in >>> zone_movable? I do not see any specific check for those. >> >> >>> >>> Btw. why is this a proper thing to do? Problems with longterm pins are >>> not only for CMA/ZONE_MOVABLE pages. Pinned pages are not reclaimable as >>> well so there is a risk of OOMs if there are too many of them. We have >>> discussed approaches that would allow to force pin invalidation/revocation >>> at LSF/MM. Isn't that a more appropriate solution to the problem you are >>> seeing? >>> >> >> The CMA area is used on powerpc platforms to allocate guest specific page >> table (hash page table). If we don't have sufficient free pages we fail to >> allocate hash page table that result in failure to start guest. >> >> Now with vfio, we end up pinning the entire guest RAM. There is a >> possibility that these guest RAM pages got allocated from CMA region. We >> already do supporting migrating those pages out except for compound pages. >> What this patch does is to start supporting compound page migration that got >> allocated out of CMA region (ie, THP pages and hugetlb pages if platform >> supported hugetlb migration). > > This definitely belongs to the changelog. > >> Now to do that I added a helper get_user_pages_cma_migrate(). >> >> I agree that long term pinned pages do have other issues. The patchset is >> not solving that issue. > > It would be great to note why a generic approach is not viable. I assume > the main reason is that those pins are pretty much permanent for the > guest lifetime so the situation has to be handled in advance. In other > words, more information please. > That is correct. I will add these details to commit message. And will also do a cover letter for the patch series. -aneesh