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 m5qLsiWY 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Correction usually happens on the fly and adds latency overhead >>> 2. Not-fully-proved theory states excessive correctable memory >>> errors can develop into uncorrectable memory error. >> Thanks for your patch. > Thanks Miaohe, sorry I missed your message (Gmail mistakenly put it in > my spam folder). > >>> Soft offline is kernel's additional solution for memory pages >>> having (excessive) corrected memory errors. Impacted page is migrated >>> to healthy page if it is in use, then the original page is discarded >>> for any future use. >>> >>> The actual policy on whether (and when) to soft offline should be >>> maintained by userspace, especially in case of HugeTLB hugepages. >>> Soft-offline dissolves a hugepage, either in-use or free, into >>> chunks of 4K pages, reducing HugeTLB pool capacity by 1 hugepage. >>> If userspace has not acknowledged such behavior, it may be surprised >>> when later mmap hugepages MAP_FAILED due to lack of hugepages. >> For in use hugetlb folio case, migrate_pages() is called. The hugetlb pool >> capacity won't be modified in that case. So I assume you're referring to the > I don't think so. > > For in-use hugetlb folio case, after migrate_pages, kernel will > dissolve_free_hugetlb_folio the src hugetlb folio. At this point > refcount of src hugetlb folio should be zero already, and > remove_hugetlb_folio will reduce the hugetlb pool capacity (both > nr_hugepages and free_hugepages) accordingly. > > For the free hugetlb folio case, dissolving also happens. But CE on > free pages should be very rare (since no one is accessing except > patrol scrubber). > > One of my test cases in patch 2/3 validates my point: the test case > MADV_SOFT_OFFLINE a mapped page and at the point soft offline > succeeds, both nr_hugepages and nr_freepages are reduced by 1. > >> free hugetlb folio case? The Hugetlb pool capacity is reduced in that case. >> But if we don't do that, we might encounter uncorrectable memory error later > If your concern is more correctable error will develop into more > severe uncorrectable, your concern is absolutely valid. There is a > tradeoff between reliability vs performance (availability of hugetlb > pages), but IMO should be decided by userspace. > >> which will be more severe? Will it be better to add a way to compensate the >> capacity? > Corner cases: What if finding physically contiguous memory takes too > long? What if we can't find any physically contiguous memory to > compensate? (then hugetlb pool will still need to be reduced). > > If we treat "compensate" as an improvement to the overall soft offline > process, it is something we can do in future and it is something > orthogonal to this control API, right? I think if userspace explicitly > tells kernel to soft offline, then they are also well-prepared for the > corner cases above. > >>> In addition, discarding the entire 1G memory page only because of >>> corrected memory errors sounds very costly and kernel better not >>> doing under the hood. But today there are at least 2 such cases: >>> 1. GHES driver sees both GHES_SEV_CORRECTED and >>> CPER_SEC_ERROR_THRESHOLD_EXCEEDED after parsing CPER. >>> 2. RAS Correctable Errors Collector counts correctable errors per >>> PFN and when the counter for a PFN reaches threshold >>> In both cases, userspace has no control of the soft offline performed >>> by kernel's memory failure recovery. >> Userspace can figure out the hugetlb folio pfn range by using `page-types -b huge >> -rlN` and then decide whether to soft offline the page according to it. But for >> GHES driver, I think it has to be done in the kernel. So add a control in /sys/ >> seems like a good idea. > Thanks. > >>> This patch series give userspace the control of soft-offlining >>> HugeTLB pages: kernel only soft offlines hugepage if userspace has >>> opt-ed in for that specific hugepage size, and exposed to userspace >>> by a new sysfs entry called softoffline_corrected_errors under >>> /sys/kernel/mm/hugepages/hugepages-${size}kB directory: >>> * When softoffline_corrected_errors=0, skip soft offlining for all >>> hugepages of size ${size}kB. >>> * When softoffline_corrected_errors=1, soft offline as before this >> Will it be better to be called as "soft_offline_corrected_errors" or simplify "soft_offline_enabled"? > "soft_offline_enabled" is less optimal as it can't be extended to > support something like "soft offline this PFN if something repeatedly > requested soft offline this exact PFN x times". (although I don't > think we need it). The "x time" thing is a threshold thing, and if your typical application needs to have a say about performance(and maintaining physically contiguous memory) over RAS, shouldn't that be baked into the driver rather than hugetlbfs ? Also, I am not comfortable with this being hugetlbfs specific. What is the objection to creating a "soft_offline_enabled" switch that is applicable to any user page size? thanks, -jane > > softoffline_corrected_errors is one char less, but if you insist, > soft_offline_corrected_errors also works for me. > >> Thanks. >> . >>