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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Oct 13, 2025 at 03:14:32PM -0700, Jiaqi Yan wrote: > On Fri, Sep 19, 2025 at 8:58 AM “William Roche wrote: > > > > From: William Roche > > > > Hello, > > > > The possibility to keep a VM using large hugetlbfs pages running after a memory > > error is very important, and the possibility described here could be a good > > candidate to address this issue. > > Thanks for expressing interest, William, and sorry for getting back to > you so late. > > > > > So I would like to provide my feedback after testing this code with the > > introduction of persistent errors in the address space: My tests used a VM > > running a kernel able to provide MFD_MF_KEEP_UE_MAPPED memfd segments to the > > test program provided with this project. But instead of injecting the errors > > with madvise calls from this program, I get the guest physical address of a > > location and inject the error from the hypervisor into the VM, so that any > > subsequent access to the location is prevented directly from the hypervisor > > level. > > This is exactly what VMM should do: when it owns or manages the VM > memory with MFD_MF_KEEP_UE_MAPPED, it is then VMM's responsibility to > isolate guest/VCPUs from poisoned memory pages, e.g. by intercepting > such memory accesses. > > > > > Using this framework, I realized that the code provided here has a problem: > > When the error impacts a large folio, the release of this folio doesn't isolate > > the sub-page(s) actually impacted by the poison. __rmqueue_pcplist() can return > > a known poisoned page to get_page_from_freelist(). > > Just curious, how exactly you can repro this leaking of a known poison > page? It may help me debug my patch. > > > > > This revealed some mm limitations, as I would have expected that the > > check_new_pages() mechanism used by the __rmqueue functions would filter these > > pages out, but I noticed that this has been disabled by default in 2023 with: > > [PATCH] mm, page_alloc: reduce page alloc/free sanity checks > > https://lore.kernel.org/all/20230216095131.17336-1-vbabka@suse.cz > > Thanks for the reference. I did turned on CONFIG_DEBUG_VM=y during dev > and testing but didn't notice any WARNING on "bad page"; It is very > likely I was just lucky. > > > > > > > This problem seems to be avoided if we call take_page_off_buddy(page) in the > > filemap_offline_hwpoison_folio_hugetlb() function without testing if > > PageBuddy(page) is true first. > > Oh, I think you are right, filemap_offline_hwpoison_folio_hugetlb > shouldn't call take_page_off_buddy(page) depend on PageBuddy(page) or > not. take_page_off_buddy will check PageBuddy or not, on the page_head > of different page orders. So maybe somehow a known poisoned page is > not taken off from buddy allocator due to this? Maybe it's the case where the poisoned page is merged to a larger page, and the PGTY_buddy flag is set on its buddy of the poisoned page, so PageBuddy() returns false?: [ free page A ][ free page B (poisoned) ] When these two are merged, then we set PGTY_buddy on page A but not on B. But even after fixing that we need to fix the race condition. > Let me try to fix it in v2, by the end of the week. If you could test > with your way of repro as well, that will be very helpful! > > > But according to me it leaves a (small) race condition where a new page > > allocation could get a poisoned sub-page between the dissolve phase and the > > attempt to remove it from the buddy allocator. > > > > I do have the impression that a correct behavior (isolating an impacted > > sub-page and remapping the valid memory content) using large pages is > > currently only achieved with Transparent Huge Pages. > > If performance requires using Hugetlb pages, than maybe we could accept to > > loose a huge page after a memory impacted MFD_MF_KEEP_UE_MAPPED memfd segment > > is released ? If it can easily avoid some other corruption. > > > > I'm very interested in finding an appropriate way to deal with memory errors on > > hugetlbfs pages, and willing to help to build a valid solution. This project > > showed a real possibility to do so, even in cases where pinned memory is used - > > with VFIO for example. > > > > I would really be interested in knowing your feedback about this project, and > > if another solution is considered more adapted to deal with errors on hugetlbfs > > pages, please let us know. > > There is also another possible path if VMM can change to back VM > memory with *1G guest_memfd*, which wraps 1G hugetlbfs. In Ackerley's > work [1], guest_memfd can split the 1G page for conversions. If we > re-use the splitting for memory failure recovery, we can probably > achieve something generally similar to THP's memory failure recovery: > split 1G to 2M and 4k chunks, then unmap only 4k of poisoned page. We > still lose the 1G TLB size so VM may be subject to some performance > sacrifice. > [1] https://lore.kernel.org/linux-mm/2ae41e0d80339da2b57011622ac2288fed65cd01.1747264138.git.ackerleytng@google.com I want to take a closer look at the actual patches but either way sounds good to me. By the way, please Cc me in future revisions :) Thanks! -- Cheers, Harry / Hyeonggon