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[2003:cb:c731:9c00:2aaa:88a4:e41d:65ba]) by smtp.gmail.com with ESMTPSA id bd12-20020a05600c1f0c00b0041b6f63f42esm6081579wmb.48.2024.04.26.23.58.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Apr 2024 23:58:27 -0700 (PDT) Message-ID: <0a9ebdaa-9346-49f4-b568-d0dce72d3108@redhat.com> Date: Sat, 27 Apr 2024 08:58:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 1/3] mm/gup: consistently name GUP-fast functions To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mike Rapoport , Jason Gunthorpe , John Hubbard , linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-riscv@lists.infradead.org, x86@kernel.org References: <20240402125516.223131-1-david@redhat.com> <20240402125516.223131-2-david@redhat.com> <8b42a24d-caf0-46ef-9e15-0f88d47d2f21@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 26.04.24 23:58, Peter Xu wrote: > On Fri, Apr 26, 2024 at 11:33:08PM +0200, David Hildenbrand wrote: >> I raised this topic in the past, and IMHO we either (a) never should have >> added COW support; or (b) added COW support by using ordinary anonymous >> memory (hey, partial mappings of hugetlb pages! ;) ). >> >> After all, COW is an optimization to speed up fork and defer copying. It >> relies on memory overcommit, but that doesn't really apply to hugetlb, so we >> fake it ... > > Good summary. > >> >> One easy ABI break I had in mind was to simply *not* allow COW-sharing of >> anon hugetlb folios; for example, simply don't copy the page into the child. >> Chances are there are not really a lot of child processes that would fail >> ... but likely we would break *something*. So there is no easy way out :( > > Right, not easy. The thing is this is one spot out of many of the > specialties, it also may or may not be worthwhile to have dedicated time > while nobody yet has a problem with it. It might be easier to start with > v2, even though that's also hard to nail everything properly - the > challenge can come from different angles. > > Thanks for the sharings, helpful. I'll go ahead with the Power fix on > hugepd putting this aside. Yes, hopefully we already do have a test case for that. When writing gup_longterm.c I was more focusing on memfd vs. ordinary file systems ("filesystem type") than how it's mapped into the page tables. > > I hope that before the end of this year, whatever I'll fix can go away, by > removing hugepd completely from Linux. For now that may or may not be as > smooth, so we'd better still fix it. Crossing fingers, I'm annoyed whenever I stumble over it :) -- Cheers, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel