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[2003:cb:c71b:5d00:18d7:1475:24bc:2417]) by smtp.gmail.com with ESMTPSA id h15-20020adfa4cf000000b003333040e7f3sm1022671wrb.60.2023.12.07.02.03.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Dec 2023 02:04:00 -0800 (PST) Message-ID: <1dcd6985-aa29-4df7-a7cb-ef57ae658861@redhat.com> Date: Thu, 7 Dec 2023 11:03:58 +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: Ryan Roberts , Steven Price , 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> <8c7f1a2f-57d2-4f20-abb2-394c7980008e@redhat.com> <5de66ff5-b6c8-4ffc-acd9-59aec4604ca4@redhat.com> <71c4b8b2-512a-4e50-9160-6ee77a5ec0a4@arm.com> <679a144a-db47-4d05-bbf7-b6a0514f5ed0@arm.com> <8aa8f095-1840-4a2e-ad06-3f375282ab6a@arm.com> <7065bbd3-64b3-4cd6-a2cd-146c556aac66@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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I'm also not sure what it buys us - instead of reading a per-page >>> flag we now have to read 128 bytes of tag for each page and check its zero. >> >> My point is, if that is the corner case, we might not care about that. > > Hi David, Hi! > my understanding is that this is NOT a corner. Alternatively, it is > really a common case. If it happens with < 1% of all large folios on swapout/swapin, it's not the common case. Even if some scenarios you point out below can and will happen. > > 1. a large folio can be partially unmapped when it is in swapche and > after it is swapped out > in all cases, its tags can be partially invalidated. I don't think > this is a corner case, as long > as userspaces are still working at the granularity of basepages, this > is always going to > happen. For example, userspace libc such as jemalloc can identify > PAGESIZE, and use > madvise(DONTNEED) to return memory to the kernel. Heap management is > still working > at the granularity of the basepage. > > 2. mprotect on a part of a large folio as Steven pointed out. > > 3.long term, we are working to swap-in large folios as a whole[1] just > like swapping out large > folios as a whole. for those ptes which are still contiguous swap > entries, i mean, which > are not unmapped by userspace after the large folios are swapped out > to swap devices, > we have a chance to swap in a whole large folio, we do have a chance > to restore tags > for the large folio without early-exit. but we still have a good > chance to fall back to base > page if we fail to allocate large folio, in this case, do_swap_page() > still works at the > granularity of basepage. and do_swap_page() will call swap_free(entry), tags of > > this particular page can be invalidated as a result. I don't immediately see how that relates. You get a fresh small folio and simply load that tag from the internal datastructure. No messing with large folios required, because you don't have a large folio. So no considerations about large folio batch MTE tag restore apply. > > 4. too many early-exit might be negative to performance. > > > So I am thinking that in the future, we need two helpers, > 1. void __arch_swap_restore(swp_entry_t entry, struct page *page); > this is always needed to support page-level tag restore. > > 2. void arch_swap_restore(swp_entry_t entry, struct folio *folio); > this can be a helper when we are able to swap in a whole folio. two > conditions must be met > (a). PTEs entries are still contiguous swap entries just as when large > folios were swapped > out. > (b). we succeed in the allocation of a large folio in do_swap_page. > > For this moment, we only need 1; we will add 2 in swap-in large folio series. > > What do you think? I agree that it's better to keep it simple for now. -- Cheers, David / dhildenb