Messages in this thread | | | Subject | Re: [RFC PATCH V2 4/4] powerpc/mm/iommu: Allow migration of cma allocated pages during mm_iommu_get | From | "Aneesh Kumar K.V" <> | Date | Fri, 7 Sep 2018 16:45:09 +0530 |
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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
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