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[2003:cb:c718:1300:9860:66a2:fe4d:c379]) by smtp.gmail.com with ESMTPSA id fc9-20020a05600c524900b004162a9f03a6sm15928122wmb.7.2024.04.08.02.43.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Apr 2024 02:43:08 -0700 (PDT) Message-ID: <7d3d3c58-1bb1-4d70-9ae3-fa96f6d624d8@redhat.com> Date: Mon, 8 Apr 2024 11:43:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache() To: Ryan Roberts , Andrew Morton , Matthew Wilcox , Huang Ying , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang , Barry Song <21cnbao@gmail.com>, Chris Li , Lance Yang Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240403114032.1162100-1-ryan.roberts@arm.com> <20240403114032.1162100-3-ryan.roberts@arm.com> <051052af-3b56-4290-98d3-fd5a1eb11ce1@redhat.com> <73adae65-4429-41d7-bbb6-4c58156060d3@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <73adae65-4429-41d7-bbb6-4c58156060d3@arm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: BEA11A0012 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: q6orgyqq57ps6y6b5wmz6tz8pffxhga4 X-HE-Tag: 1712569392-231612 X-HE-Meta: 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 P4KAw5I4 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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: >>> + >>> +/** >>> + * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries >>> + * @start_ptep: Page table pointer for the first entry. >>> + * @max_nr: The maximum number of table entries to consider. >>> + * @entry: Swap entry recovered from the first table entry. >>> + * >>> + * Detect a batch of contiguous swap entries: consecutive (non-present) PTEs >>> + * containing swap entries all with consecutive offsets and targeting the same >>> + * swap type. >>> + * >> >> Likely you should document that any swp pte bits are ignored? () > > Sorry I don't understand this comment. I thought any non-none, non-present PTE > was always considered to contain only a "swap entry" and a swap entry consists > of a "type" and an "offset" only. (and its a special "non-swap" swap entry if > type > SOME_CONSTANT) Are you saying there are additional fields in the PTE that > are not part of the swap entry? pte_swp_soft_dirty() pte_swp_clear_exclusive() pte_swp_uffd_wp() Are PTE bits used for swp PTE. There is also dirty/young for migration entries, but that's not of a concern here, because we stop for non_swap_entry(). > > >> >>> + * max_nr must be at least one and must be limited by the caller so scanning >>> + * cannot exceed a single page table. >>> + * >>> + * Return: the number of table entries in the batch. >>> + */ >>> +static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, >>> +                 swp_entry_t entry) >>> +{ >>> +    const pte_t *end_ptep = start_ptep + max_nr; >>> +    unsigned long expected_offset = swp_offset(entry) + 1; >>> +    unsigned int expected_type = swp_type(entry); >>> +    pte_t *ptep = start_ptep + 1; >>> + >>> +    VM_WARN_ON(max_nr < 1); >>> +    VM_WARN_ON(non_swap_entry(entry)); >>> + >>> +    while (ptep < end_ptep) { >>> +        pte_t pte = ptep_get(ptep); >>> + >>> +        if (pte_none(pte) || pte_present(pte)) >>> +            break; >>> + >>> +        entry = pte_to_swp_entry(pte); >>> + >>> +        if (non_swap_entry(entry) || >>> +            swp_type(entry) != expected_type || >>> +            swp_offset(entry) != expected_offset) >>> +            break; >>> + >>> +        expected_offset++; >>> +        ptep++; >>> +    } >>> + >>> +    return ptep - start_ptep; >>> +} >> >> Looks very clean :) >> >> I was wondering whether we could similarly construct the expected swp PTE and >> only check pte_same. >> >> expected_pte = __swp_entry_to_pte(__swp_entry(expected_type, expected_offset)); >> >> ... or have a variant to increase only the swp offset for an existing pte. But >> non-trivial due to the arch-dependent format. >> >> But then, we'd fail on mismatch of other swp pte bits. > > Hmm, perhaps I have a misunderstanding regarding "swp pte bits"... > >> >> >> On swapin, when reusing this function (likely!), we'll might to make sure that >> the PTE bits match as well. >> >> See below regarding uffd-wp. >> >> >>>   #endif /* CONFIG_MMU */ >>>     void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio, >>> diff --git a/mm/madvise.c b/mm/madvise.c >>> index 1f77a51baaac..070bedb4996e 100644 >>> --- a/mm/madvise.c >>> +++ b/mm/madvise.c >>> @@ -628,6 +628,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned >>> long addr, >>>       struct folio *folio; >>>       int nr_swap = 0; >>>       unsigned long next; >>> +    int nr, max_nr; >>>         next = pmd_addr_end(addr, end); >>>       if (pmd_trans_huge(*pmd)) >>> @@ -640,7 +641,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned >>> long addr, >>>           return 0; >>>       flush_tlb_batched_pending(mm); >>>       arch_enter_lazy_mmu_mode(); >>> -    for (; addr != end; pte++, addr += PAGE_SIZE) { >>> +    for (; addr != end; pte += nr, addr += PAGE_SIZE * nr) { >>> +        nr = 1; >>>           ptent = ptep_get(pte); >>>             if (pte_none(ptent)) >>> @@ -655,9 +657,11 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned >>> long addr, >>>                 entry = pte_to_swp_entry(ptent); >>>               if (!non_swap_entry(entry)) { >>> -                nr_swap--; >>> -                free_swap_and_cache(entry); >>> -                pte_clear_not_present_full(mm, addr, pte, tlb->fullmm); >>> +                max_nr = (end - addr) / PAGE_SIZE; >>> +                nr = swap_pte_batch(pte, max_nr, entry); >>> +                nr_swap -= nr; >>> +                free_swap_and_cache_nr(entry, nr); >>> +                clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm); >>>               } else if (is_hwpoison_entry(entry) || >>>                      is_poisoned_swp_entry(entry)) { >>>                   pte_clear_not_present_full(mm, addr, pte, tlb->fullmm); >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 7dc6c3d9fa83..ef2968894718 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -1637,12 +1637,13 @@ static unsigned long zap_pte_range(struct mmu_gather >>> *tlb, >>>                   folio_remove_rmap_pte(folio, page, vma); >>>               folio_put(folio); >>>           } else if (!non_swap_entry(entry)) { >>> -            /* Genuine swap entry, hence a private anon page */ >>> +            max_nr = (end - addr) / PAGE_SIZE; >>> +            nr = swap_pte_batch(pte, max_nr, entry); >>> +            /* Genuine swap entries, hence a private anon pages */ >>>               if (!should_zap_cows(details)) >>>                   continue; >>> -            rss[MM_SWAPENTS]--; >>> -            if (unlikely(!free_swap_and_cache(entry))) >>> -                print_bad_pte(vma, addr, ptent, NULL); >>> +            rss[MM_SWAPENTS] -= nr; >>> +            free_swap_and_cache_nr(entry, nr); >>>           } else if (is_migration_entry(entry)) { >>>               folio = pfn_swap_entry_folio(entry); >>>               if (!should_zap_folio(details, folio)) >>> @@ -1665,8 +1666,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, >>>               pr_alert("unrecognized swap entry 0x%lx\n", entry.val); >>>               WARN_ON_ONCE(1); >>>           } >>> -        pte_clear_not_present_full(mm, addr, pte, tlb->fullmm); >>> -        zap_install_uffd_wp_if_needed(vma, addr, pte, 1, details, ptent); >>> +        clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm); >> >> For zap_install_uffd_wp_if_needed(), the uffd-wp bit has to match. >> >> zap_install_uffd_wp_if_needed() will use the uffd-wp information in >> ptent->pteval to make a decision whether to place PTE_MARKER_UFFD_WP markers. >> >> On mixture, you either lose some or place too many markers. > > What path are you concerned about here? I don't get how what you describe can > happen? swap_pte_batch() will only give me a batch of actual swap entries and > actual swap entries don't contain uffd-wp info, IIUC. If the function gets to a > "non-swap" swap entry, it bails. I thought the uffd-wp info was populated based > on the VMA state at swap-in? I think you are telling me that it's persisted > across the swap per-pte? Please see zap_install_uffd_wp_if_needed(): if (unlikely(pte_swp_uffd_wp_any(pteval))) arm_uffd_pte = true; The PTEs (swp PTEs to be precise) contain uffd-wp informtation. [...] >>> +    /* >>> +     * Short-circuit the below loop if none of the entries had their >>> +     * reference drop to zero. >>> +     */ >>> +    if (!any_only_cache) >>> +        goto out; >>>   -        count = __swap_entry_free(p, entry); >>> -        if (count == SWAP_HAS_CACHE) >>> -            __try_to_reclaim_swap(p, swp_offset(entry), >>> +    /* >>> +     * Now go back over the range trying to reclaim the swap cache. This is >>> +     * more efficient for large folios because we will only try to reclaim >>> +     * the swap once per folio in the common case. If we do >>> +     * __swap_entry_free() and __try_to_reclaim_swap() in the same loop, the >>> +     * latter will get a reference and lock the folio for every individual >>> +     * page but will only succeed once the swap slot for every subpage is >>> +     * zero. >>> +     */ >>> +    for (offset = swp_offset(entry); offset < end; offset += nr) { >>> +        nr = 1; >>> +        if (READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) { >> >> Here we use READ_ONCE() only, above data_race(). Hmmm. > > Yes. I think this is correct. > > READ_ONCE() is a "marked access" which KCSAN understands, so it won't complain > about it. So data_race() isn't required when READ_ONCE() (or WRITE_ONCE()) is > used. I believe READ_ONCE() is required here because we don't have a lock and we > want to make sure we read it in a non-tearing manner. > > We don't need the READ_ONCE() above since we don't care about the exact value - > only that it's not 0 (because we should be holding a ref). So do a plain access > to give the compiler a bit more freedom. But we need to mark that with > data_race() to stop KCSAN from complaining. Okay. -- Cheers, David / dhildenb