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[2003:cb:c71c:a700:bba7:849a:ecf1:5404]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-385df74157asm1559438f8f.0.2024.11.29.06.48.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 29 Nov 2024 06:48:34 -0800 (PST) Message-ID: Date: Fri, 29 Nov 2024 15:48:33 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] perf: map pages in advance To: Lorenzo Stoakes Cc: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Kan Liang , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox References: <9d6be5bd-ffb9-4a27-b56d-521cf6b7486e@redhat.com> <6cab3e8a-dff7-41d1-af22-f18b8f2820dc@lucifer.local> <94dabe57-232b-4a21-b2cf-bcfbda75c881@lucifer.local> <6795cc9a-6230-431f-b089-7909f7bc4f30@redhat.com> <60191c97-dce2-4a92-8b47-c402478ba336@lucifer.local> <9d4ef1a2-11fb-455f-8b37-954215bf25d2@redhat.com> <14895682-a013-419f-bee8-1476540ddedd@lucifer.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: C85EE180008 X-Stat-Signature: wesefsrd4jdappc6cq9cp7wt4acim988 X-Rspam-User: X-HE-Tag: 1732891714-406251 X-HE-Meta: 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 VgQc4IeO Yv3W39BPV6eA4slpy5ah3GNSDYKWMMOdidERWmdoXzKfa9EWgM2vDpndp173RWfsmIgBTDXlACKUvnJbu4dP1dHQDe8a5phdZHHiGK7tPfDhwkIiUgI3Yzble2XdaOb2uT4czJlddJlTBxjp7+alnI3ytKYnVuNbi1IBPiDHxbpeG6fG07JQNUM2Ab9XJAQ/VBUdOI9FAbDA6Uo0Bhfo3crvEKV7nKiEhzIUbJ+R1ozx1NfUcYR7qo69MduFxsgTv10cQLMVh/o2s/1cyhSMPCbdmRPSu017KiWy2kKq/M1J2Ut563IHKfcb9ibYL8BhbWw6IdR9W9GWWkffBZ8jvrNrCZNKhulUyR0MIgOLwLZFP4bqyQBLFvCgzx3zSYddbyR47xDcwcGSQHDFM5S/2gwKhfA== 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 29.11.24 15:35, Lorenzo Stoakes wrote: > On Fri, Nov 29, 2024 at 03:09:49PM +0100, David Hildenbrand wrote: >>>>> I just tested it and write() works fine, it uses uaccess afaict as part of the >>>>> lib/iov_iter.c code: >>>>> >>>>> generic_perform_write() >>>>> -> copy_folio_from_iter_atomic() >>>>> -> copy_page_from_iter_atomic() >>>>> -> __copy_from_iter() >>>>> -> copy_from_user_iter() >>>>> -> raw_copy_from_user() >>>>> -> copy_user_generic() >>>>> -> [uaccess asm] >>>>> >>>> >>>> Ah yes. O_DIRECT is the problematic bit I suspect, which will use GUP. >>>> >>>> Ptrace access and friends should also no longer work on these pages, likely >>>> that's tolerable. >>> >>> Yeah Peter can interject if not, but I'd be _very_ surprised if anybody expects >>> to be able to ptrace perf counter mappings in another process (it'd be kind of >>> totally insane to do that anyway since it's a ring buffer that needs special >>> handing anyway). >> >> I think so as well. Disallowing GUP has some side effects, like not getting >> these pages included in a coredump etc ... at least I hope nobody uses >> O_DIRECT on them. > > We set VM_DONTDUMP anyway (set by remap_pfn_range() also) so this part won't be > a problem, and I can't see anybody using O_DIRECT on them, sensibly. Ah, even better. >> >> Actually, the whole thing is on my todo list, because messing with the RMAP >> with vm_insert_pages() doesn't make any sense (whereby the refcount might!). >> See the TODO I added in __folio_rmap_sanity_checks(). > > How long is your TODO list I wonder? :)) I imagine it requires a huge page to > map at this point.. :D Too long, but as some of these TODOs stand in the memdesc way, fortunately other people might be able to give a helping hand at some point ;) I'll play with using a page type for some of these "simple" cases and see how hard it will get. > >> >> -- >> Cheers, >> >> David / dhildenb >> > > Cool will respin and send out shortly with an appropriately 'forgive us father > for we have sinned' comment. > > Thanks for taking the time to discuss this! Much appreciated. Thanks for bearing with me. Whenever PFNMAP/MIXEDMAP is involved it gets tricky, and as soon as PFNMAP/MIXEDMAP is inevitable, things get ugly. :) -- Cheers, David / dhildenb