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[2003:cb:c721:4400:d83c:42bd:326a:8b05]) by smtp.gmail.com with ESMTPSA id y18-20020adfee12000000b003200c918c81sm2008508wrn.112.2023.11.17.03.28.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Nov 2023 03:28:08 -0800 (PST) Message-ID: <8c7f1a2f-57d2-4f20-abb2-394c7980008e@redhat.com> Date: Fri, 17 Nov 2023 12:28:06 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios To: Barry Song <21cnbao@gmail.com> Cc: steven.price@arm.com, akpm@linux-foundation.org, ryan.roberts@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, shy828301@gmail.com, v-songbaohua@oppo.com, wangkefeng.wang@huawei.com, willy@infradead.org, xiang@kernel.org, ying.huang@intel.com, yuzhao@google.com References: <20231114014313.67232-1-v-songbaohua@oppo.com> <864489b3-5d85-4145-b5bb-5d8a74b9b92d@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: B7B6D4000E X-Rspam-User: X-Stat-Signature: baskg5d1wi8m1yhhi7ybc1d5n4sckhu9 X-Rspamd-Server: rspam01 X-HE-Tag: 1700220492-728185 X-HE-Meta: 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 52vcTMmv 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 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 17.11.23 01:15, Barry Song wrote: > On Fri, Nov 17, 2023 at 7:47 AM Barry Song <21cnbao@gmail.com> wrote: >> >> On Thu, Nov 16, 2023 at 5:36 PM David Hildenbrand wrote: >>> >>> On 15.11.23 21:49, Barry Song wrote: >>>> On Wed, Nov 15, 2023 at 11:16 PM David Hildenbrand wrote: >>>>> >>>>> On 14.11.23 02:43, Barry Song wrote: >>>>>> This patch makes MTE tags saving and restoring support large folios, >>>>>> then we don't need to split them into base pages for swapping out >>>>>> on ARM64 SoCs with MTE. >>>>>> >>>>>> arch_prepare_to_swap() should take folio rather than page as parameter >>>>>> because we support THP swap-out as a whole. >>>>>> >>>>>> Meanwhile, arch_swap_restore() should use page parameter rather than >>>>>> folio as swap-in always works at the granularity of base pages right >>>>>> now. >>>>> >>>>> ... but then we always have order-0 folios and can pass a folio, or what >>>>> am I missing? >>>> >>>> Hi David, >>>> you missed the discussion here: >>>> >>>> https://lore.kernel.org/lkml/CAGsJ_4yXjex8txgEGt7+WMKp4uDQTn-fR06ijv4Ac68MkhjMDw@mail.gmail.com/ >>>> https://lore.kernel.org/lkml/CAGsJ_4xmBAcApyK8NgVQeX_Znp5e8D4fbbhGguOkNzmh1Veocg@mail.gmail.com/ >>> >>> Okay, so you want to handle the refault-from-swapcache case where you get a >>> large folio. >>> >>> I was mislead by your "folio as swap-in always works at the granularity of >>> base pages right now" comment. >>> >>> What you actually wanted to say is "While we always swap in small folios, we >>> might refault large folios from the swapcache, and we only want to restore >>> the tags for the page of the large folio we are faulting on." >>> >>> But, I do if we can't simply restore the tags for the whole thing at once >>> at make the interface page-free? >>> >>> Let me elaborate: >>> >>> IIRC, if we have a large folio in the swapcache, the swap entries/offset are >>> contiguous. If you know you are faulting on page[1] of the folio with a >>> given swap offset, you can calculate the swap offset for page[0] simply by >>> subtracting from the offset. >>> >>> See page_swap_entry() on how we perform this calculation. >>> >>> >>> So you can simply pass the large folio and the swap entry corresponding >>> to the first page of the large folio, and restore all tags at once. >>> >>> So the interface would be >>> >>> arch_prepare_to_swap(struct folio *folio); >>> void arch_swap_restore(struct page *folio, swp_entry_t start_entry); >>> >>> I'm sorry if that was also already discussed. >> >> This has been discussed. Steven, Ryan and I all don't think this is a good >> option. in case we have a large folio with 16 basepages, as do_swap_page >> can only map one base page for each page fault, that means we have >> to restore 16(tags we restore in each page fault) * 16(the times of page faults) >> for this large folio. >> >> and still the worst thing is the page fault in the Nth PTE of large folio >> might free swap entry as that swap has been in. >> do_swap_page() >> { >> /* >> * Remove the swap entry and conditionally try to free up the swapcache. >> * We're already holding a reference on the page but haven't mapped it >> * yet. >> */ >> swap_free(entry); >> } >> >> So in the page faults other than N, I mean 0~N-1 and N+1 to 15, you might access >> a freed tag. > > And David, one more information is that to keep the parameter of > arch_swap_restore() unchanged as folio, > i actually tried an ugly approach in rfc v2: > > +void arch_swap_restore(swp_entry_t entry, struct folio *folio) > +{ > + if (system_supports_mte()) { > + /* > + * We don't support large folios swap in as whole yet, but > + * we can hit a large folio which is still in swapcache > + * after those related processes' PTEs have been unmapped > + * but before the swapcache folio is dropped, in this case, > + * we need to find the exact page which "entry" is mapping > + * to. If we are not hitting swapcache, this folio won't be > + * large > + */ > + struct page *page = folio_file_page(folio, swp_offset(entry)); > + mte_restore_tags(entry, page); > + } > +} > > And obviously everybody in the discussion hated it :-) > I can relate :D > i feel the only way to keep API unchanged using folio is that we > support restoring PTEs > all together for the whole large folio and we support the swap-in of > large folios. This is > in my list to do, I will send a patchset based on Ryan's large anon > folios series after a > while. till that is really done, it seems using page rather than folio > is a better choice. I think just restoring all tags and remembering for a large folio that they have been restored might be the low hanging fruit. But as always, devil is in the detail :) -- Cheers, David / dhildenb