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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1763569900; bh=1l5JKzzB/X/aOY4SRfPTDUqyy1NGJefVlVFQlGnGZ6Y=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=TmaDy11SGNssyolc2WtbIKKTe8xhNDd843YXydYaEuj9vMLmiZt6HRZ6k7ozQkAN0 THsHhXPKb8i7tuV7KStL6maJD0F78ec3xLR/MjRdHCDmerCpvjmmNdLXnB26znxqKT i/419SDSuqqkWRlZjx4MgJNqBG74vpICCIecuAOZ+x9eZ+fCccmbf4Mvj4Qyt7cmiM si/X0tYEDwx7i4ZbCJqbkledCQlFl1yJ/aWhTOEANUKnJzei581xkiWCYKLagHsJAD p5B1s7Dnda+d57w57xauHyHAoJHyNGI4RlyAyr0kC9frMbAIo1MxGh2SvSj3a9azcG Wc9Pa1Tg8spUg== Message-ID: Date: Wed, 19 Nov 2025 17:31:35 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Bug: Performance regression in 1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race From: "David Hildenbrand (Red Hat)" To: Lorenzo Stoakes 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" References: <81d096fb-f2c2-4b26-ab1b-486001ee2cac@lucifer.local> <4ebbd082-86e3-4b86-bb01-6325f300fc9c@lucifer.local> <2bff49c4-6292-446b-9cd4-1563358fe3b4@redhat.com> <0dabc80e-9c68-41be-b936-8c6e55582c79@lucifer.local> <944a09b0-77a6-40c9-8bea-d6b86a438d8a@kernel.org> <1d53ef79-c88c-4c5b-af82-1eb22306993b@lucifer.local> <968d5458-7d2b-4a8d-a2a6-0931cd87898f@kernel.org> Content-Language: en-US In-Reply-To: <968d5458-7d2b-4a8d-a2a6-0931cd87898f@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 52B4214000E X-Stat-Signature: thbwbwtrdmrep6xw7ewqi7hb3y9beacb X-HE-Tag: 1763569901-849960 X-HE-Meta: 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 /BOESxmp 0wXmZU46AGLFHg/fzNBujfSklvGaF0lCzzyog1e/eS3dZrkXdTaQ+rGlDI7tezDuZ5z9/NKLb2L5LsuS5IPU+cduXjs4RyqQFfS5Gzvdn1dF4j9I3Bm9BMLm4b4FB3eCAQpiZuTLNP2GRFangud1f0oG8HccValZKoLR16+dfry+Y1Y1MtP9j/aQufEzITKmdYwt6y2ol2KUSjrym5DItcJ8g8ybPevdTNGrqf1eE8is9aT7/pUHMAjGMpxN9JRWquSDIxrAapjkZ81eF1xTF2xJ7tQCYPNbQRo+lNuQF60gq4LAvBnPTgtvyXdrJDnnIw839Y6F9rwTlumCsf5+z5O2mkwEd8eO9u0E1 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 19.11.25 17:29, David Hildenbrand (Red Hat) wrote: >>> >>> So what I am currently looking into is simply reducing (batching) the number >>> of IPIs. >> >> As in the IPIs we are now generating in tlb_remove_table_sync_one()? >> >> Or something else? > > Yes, for now. I'm essentially reducing the number of > tlb_remove_table_sync_one() calls. > >> >> As this bug is only an issue when we don't use IPIs for pgtable freeing right >> (e.g. CONFIG_MMU_GATHER_RCU_TABLE_FREE is set), as otherwise >> tlb_remove_table_sync_one() is a no-op? > > Right. But it's still confusing: I think for page table unsharing we > always need an IPI one way or the other to make sure GUP-fast was called. > > At least for preventing that anybody would be able to reuse the page > table in the meantime. > > That is either: > > (a) The TLB shootdown implied an IPI > > (b) We manually send one > > But that's where it gets confusing: nowadays x86 also selects > MMU_GATHER_RCU_TABLE_FREE, meaning we would get a double IPI? > > This is so complicated, so I might be missing something. > > But it's the same behavior we have in collapse_huge_page() where we first ... flush and then call tlb_remove_table_sync_one(). -- Cheers David