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[2003:cb:c721:a000:7426:f6b4:82a3:c6ab]) by smtp.gmail.com with ESMTPSA id c9-20020adfef49000000b0032fb7b4f191sm3768908wrp.91.2023.11.24.00.55.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 24 Nov 2023 00:55:11 -0800 (PST) Message-ID: <5de66ff5-b6c8-4ffc-acd9-59aec4604ca4@redhat.com> Date: Fri, 24 Nov 2023 09:55:10 +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>, Ryan Roberts Cc: steven.price@arm.com, akpm@linux-foundation.org, 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> <8c7f1a2f-57d2-4f20-abb2-394c7980008e@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: 40D7514000D X-Rspam-User: X-Stat-Signature: ua9b77c1xni4fditsts3z3nm64mwxbeo X-Rspamd-Server: rspam01 X-HE-Tag: 1700816118-668982 X-HE-Meta: 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 twgWDOM/ 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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 24.11.23 02:35, Barry Song wrote: > On Mon, Nov 20, 2023 at 11:57 PM Ryan Roberts wrote: >> >> On 20/11/2023 09:11, David Hildenbrand wrote: >>> On 17.11.23 19:41, Barry Song wrote: >>>> On Fri, Nov 17, 2023 at 7:28 PM David Hildenbrand wrote: >>>>> >>>>> 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 :) >>>> >>>> Hi David, >>>> thanks for all your suggestions though my feeling is this is too complex and >>>> is not worth it for at least three reasons. >>> >>> Fair enough. >>> >>>> >>>> 1. In multi-thread and particularly multi-processes, we need some locks to >>>> protect and help know if one process is the first one to restore tags and if >>>> someone else is restoring tags when one process wants to restore. there >>>> is not this kind of fine-grained lock at all. >>> >>> We surely always hold the folio lock on swapin/swapout, no? So when these >>> functions are called. >>> >>> So that might just work already -- unless I am missing something important. >> >> We already have a page flag that we use to mark the page as having had its mte >> state associated; PG_mte_tagged. This is currently per-page (and IIUC, Matthew >> has been working to remove as many per-page flags as possible). Couldn't we just >> make arch_swap_restore() take a folio, restore the tags for *all* the pages and >> repurpose that flag to be per-folio (so head page only)? It looks like the the >> mte code already manages all the serialization requirements too. Then >> arch_swap_restore() can just exit early if it sees the flag is already set on >> the folio. >> >> One (probably nonsense) concern that just sprung to mind about having MTE work >> with large folios in general; is it possible that user space could cause a large >> anon folio to be allocated (THP), then later mark *part* of it to be tagged with >> MTE? In this case you would need to apply tags to part of the folio only. >> Although I have a vague recollection that any MTE areas have to be marked at >> mmap time and therefore this type of thing is impossible? > > right, we might need to consider only a part of folio needs to be > mapped and restored MTE tags. > do_swap_page() can have a chance to hit a large folio but it only > needs to fault-in a page. > > A case can be quite simple as below, > > 1. anon folio shared by process A and B > 2. add_to_swap() as a large folio; > 3. try to unmap A and B; > 4. after A is unmapped(ptes become swap entries), we do a > MADV_DONTNEED on a part of the folio. this can > happen very easily as userspace is still working in 4KB level; > userspace heap management can free an > basepage area by MADV_DONTNEED; > madvise(address, MADV_DONTNEED, 4KB); > 5. A refault on address + 8KB, we will hit large folio in > do_swap_page() but we will only need to map > one basepage, we will never need this DONTNEEDed in process A. > > another more complicated case can be mprotect and munmap a part of > large folios. since userspace > has no idea of large folios in their mind, they can do all strange > things. are we sure in all cases, > large folios have been splitted into small folios? To handle that, we'd have to identify a) if a subpage has an mte tag to save during swapout b) if a subpage has an mte tag to restore during swapin I suspect b) can be had from whatever datastructure we're using to actually save the tags? For a), is there some way to have that information from the HW? -- Cheers, David / dhildenb