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owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Nov 03, 2025 at 05:16:33PM +0900, Harry Yoo wrote: > On Thu, Oct 30, 2025 at 10:28:48AM -0700, Jiaqi Yan wrote: > > On Thu, Oct 30, 2025 at 4:51 AM Miaohe Lin wrote: > > > On 2025/10/28 15:00, Harry Yoo wrote: > > > > On Mon, Oct 27, 2025 at 09:17:31PM -0700, Jiaqi Yan wrote: > > > >> On Wed, Oct 22, 2025 at 6:09 AM Harry Yoo wrote: > > > >>> 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: > > > >>> But even after fixing that we need to fix the race condition. > > > >> > > > >> What exactly is the race condition you are referring to? > > > > > > > > When you free a high-order page, the buddy allocator doesn't not check > > > > PageHWPoison() on the page and its subpages. It checks PageHWPoison() > > > > only when you free a base (order-0) page, see free_pages_prepare(). > > > > > > I think we might could check PageHWPoison() for subpages as what free_page_is_bad() > > > does. If any subpage has HWPoisoned flag set, simply drop the folio. Even we could > > > > Agree, I think as a starter I could try to, for example, let > > free_pages_prepare scan HWPoison-ed subpages if the base page is high > > order. In the optimal case, HugeTLB does move PageHWPoison flag from > > head page to the raw error pages. > > [+Cc page allocator folks] > > AFAICT enabling page sanity check in page alloc/free path would be against > past efforts to reduce sanity check overhead. > > [1] https://lore.kernel.org/linux-mm/1460711275-1130-15-git-send-email-mgorman@techsingularity.net/ > [2] https://lore.kernel.org/linux-mm/1460711275-1130-16-git-send-email-mgorman@techsingularity.net/ > [3] https://lore.kernel.org/all/20230216095131.17336-1-vbabka@suse.cz > > I'd recommend to check hwpoison flag before freeing it to the buddy > when we know a memory error has occurred (I guess that's also what Miaohe > suggested). > > > > do it better -- Split the folio and let healthy subpages join the buddy while reject > > > the hwpoisoned one. > > > > > > > > > > > AFAICT there is nothing that prevents the poisoned page to be > > > > allocated back to users because the buddy doesn't check PageHWPoison() > > > > on allocation as well (by default). > > > > > > > > So rather than freeing the high-order page as-is in > > > > dissolve_free_hugetlb_folio(), I think we have to split it to base pages > > > > and then free them one by one. > > > > > > It might not be worth to do that as this would significantly increase the overhead > > > of the function while memory failure event is really rare. > > > > IIUC, Harry's idea is to do the split in dissolve_free_hugetlb_folio > > only if folio is HWPoison-ed, similar to what Miaohe suggested > > earlier. > > Yes, and if we do the check before moving HWPoison flag to raw pages, > it'll be just a single folio_test_hwpoison() call. > > > BTW, I believe this race condition already exists today when > > memory_failure handles HWPoison-ed free hugetlb page; it is not > > something introduced via this patchset. I will fix or improve this in > > a separate patchset. > > That makes sense. Wait, without this patchset, do we even free the hugetlb folio when its subpage is hwpoisoned? I don't think we do, but I'm not expert at MFR... If we don't, the mainline kernel should not be affected by this yet? > Thanks for working on this! > > > > > That way, free_pages_prepare() will catch that it's poisoned and won't > > > > add it back to the freelist. Otherwise there will always be a window > > > > where the poisoned page can be allocated to users - before it's taken > > > > off from the buddy. -- Cheers, Harry / Hyeonggon