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DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) by IA1PR10MB7333.namprd10.prod.outlook.com (2603:10b6:208:3fa::10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9203.12; Thu, 16 Oct 2025 09:21:25 +0000 Received: from DM4PR10MB8218.namprd10.prod.outlook.com ([fe80::2650:55cf:2816:5f2]) by DM4PR10MB8218.namprd10.prod.outlook.com ([fe80::2650:55cf:2816:5f2%2]) with mapi id 15.20.9228.010; Thu, 16 Oct 2025 09:21:25 +0000 Date: Thu, 16 Oct 2025 10:21:22 +0100 From: Lorenzo Stoakes To: David Hildenbrand Cc: Jann Horn , "Uschakow, Stanislav" , "linux-mm@kvack.org" , "trix@redhat.com" , "ndesaulniers@google.com" , "nathan@kernel.org" , "akpm@linux-foundation.org" , "muchun.song@linux.dev" , "mike.kravetz@oracle.com" , "liam.howlett@oracle.com" , "osalvador@suse.de" , "vbabka@suse.cz" , "stable@vger.kernel.org" Subject: Re: Bug: Performance regression in 1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race 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Hildenbrand wrote: > On 01.09.25 12:58, Jann Horn wrote: > > Hi! > > > > On Fri, Aug 29, 2025 at 4:30 PM Uschakow, Stanislav wrote: > > > We have observed a huge latency increase using `fork()` after ingesting the CVE-2025-38085 fix which leads to the commit `1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race`. On large machines with 1.5TB of memory with 196 cores, we identified mmapping of 1.2TB of shared memory and forking itself dozens or hundreds of times we see a increase of execution times of a factor of 4. The reproducer is at the end of the email. > > > > Yeah, every 1G virtual address range you unshare on unmap will do an > > extra synchronous IPI broadcast to all CPU cores, so it's not very > > surprising that doing this would be a bit slow on a machine with 196 > > cores. > > > > > My observation/assumption is: > > > > > > each child touches 100 random pages and despawns > > > on each despawn `huge_pmd_unshare()` is called > > > each call to `huge_pmd_unshare()` syncrhonizes all threads using `tlb_remove_table_sync_one()` leading to the regression > > > > Yeah, makes sense that that'd be slow. > > > > There are probably several ways this could be optimized - like maybe > > changing tlb_remove_table_sync_one() to rely on the MM's cpumask > > (though that would require thinking about whether this interacts with > > remote MM access somehow), or batching the refcount drops for hugetlb > > shared page tables through something like struct mmu_gather, or doing > > something special for the unmap path, or changing the semantics of > > hugetlb page tables such that they can never turn into normal page > > tables again. However, I'm not planning to work on optimizing this. > > I'm currently looking at the fix and what sticks out is "Fix it with an > explicit broadcast IPI through tlb_remove_table_sync_one()". > > (I don't understand how the page table can be used for "normal, > non-hugetlb". I could only see how it is used for the remaining user for > hugetlb stuff, but that's different question) Right, this surely is related only to hugetlb PTS, otherwise the refcount shouldn't be a factor no? > > How does the fix work when an architecture does not issue IPIs for TLB > shootdown? To handle gup-fast on these architectures, we use RCU. > > So I'm wondering whether we use RCU somehow. Presumably you mean whether we _can_ use RCU somehow? > > But note that in gup_fast_pte_range(), we are validating whether the PMD > changed: > > if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) || > unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) { > gup_put_folio(folio, 1, flags); > goto pte_unmap; > } Right and as per the comment there: /* ... * For THP collapse, it's a bit more complicated because GUP-fast may be * walking a pgtable page that is being freed (pte is still valid but pmd * can be cleared already). To avoid race in such condition, we need to * also check pmd here to make sure pmd doesn't change (corresponds to * pmdp_collapse_flush() in the THP collapse code path). ... */ So if this can correctly handle a cleared PMD entry in the teardown case, surely it can handle it in this case also? > > > So in case the page table got reused in the meantime, we should just back > off and be fine, right? Yeah seems to be the case to me. > > -- > Cheers > > David / dhildenb > So it seems like you have a proposal here - could you send a patch so we can assess it please? :) I'm guessing we need only consider the 'remaining user' case for hugetlb PTS right? And perhaps stabilise via RCU somehow? Cheers, Lorenzo