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[2003:cb:c733:f100:75e7:a0a4:9ac2:1abb]) by smtp.gmail.com with ESMTPSA id o17-20020a05600c4fd100b00412d68dbf75sm8668421wmq.35.2024.03.04.12.50.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Mar 2024 12:50:28 -0800 (PST) Message-ID: <65a66eb9-41f8-4790-8db2-0c70ea15979f@redhat.com> Date: Mon, 4 Mar 2024 21:50:27 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags To: Ryan Roberts , Andrew Morton , Matthew Wilcox , Huang Ying , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20231025144546.577640-1-ryan.roberts@arm.com> <20231025144546.577640-2-ryan.roberts@arm.com> <6541e29b-f25a-48b8-a553-fd8febe85e5a@redhat.com> <2934125a-f2e2-417c-a9f9-3cb1e074a44f@redhat.com> <049818ca-e656-44e4-b336-934992c16028@arm.com> <949b6c22-d737-4060-9ca1-a69d8e986d90@redhat.com> <9ed743a7-0c5d-49d9-b8b2-d58364df1f5f@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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I think _swap_info_get() would break >>> if this could race with swapoff(), and __swap_entry_free() looks up the cluster >>> from an array, which would also be freed by swapoff if racing: >>> >>> int free_swap_and_cache(swp_entry_t entry) >>> { >>>     struct swap_info_struct *p; >>>     unsigned char count; >>> >>>     if (non_swap_entry(entry)) >>>         return 1; >>> >>>     p = _swap_info_get(entry); >>>     if (p) { >>>         count = __swap_entry_free(p, entry); >> >> If count dropped to 0 and >> >>>         if (count == SWAP_HAS_CACHE) >> >> >> count is now SWAP_HAS_CACHE, there is in fact no swap entry anymore. We removed >> it. That one would have to be reclaimed asynchronously. >> >> The existing code we would call swap_page_trans_huge_swapped() with the SI it >> obtained via _swap_info_get(). >> >> I also don't see what should be left protecting the SI. It's not locked anymore, >> the swapcounts are at 0. We don't hold the folio lock. >> >> try_to_unuse() will stop as soon as si->inuse_pages is at 0. Hm ... > > But, assuming the caller of free_swap_and_cache() acquires the PTL first, I > think this all works out ok? While free_swap_and_cache() is running, > try_to_unuse() will wait for the PTL. Or if try_to_unuse() runs first, then > free_swap_and_cache() will never be called because the swap entry will have been > removed from the PTE? But can't try_to_unuse() run, detect !si->inuse_pages and not even bother about scanning any further page tables? But my head hurts from digging through that code. Let me try again: __swap_entry_free() might be the last user and result in "count == SWAP_HAS_CACHE". swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0. So the question is: could someone reclaim the folio and turn si->inuse_pages==0, before we completed swap_page_trans_huge_swapped(). Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are still references by swap entries. Process 1 still references subpage 0 via swap entry. Process 2 still references subpage 1 via swap entry. Process 1 quits. Calls free_swap_and_cache(). -> count == SWAP_HAS_CACHE [then, preempted in the hypervisor etc.] Process 2 quits. Calls free_swap_and_cache(). -> count == SWAP_HAS_CACHE Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls __try_to_reclaim_swap(). __try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->put_swap_folio()-> free_swap_slot()->swapcache_free_entries()->swap_entry_free()->swap_range_free()-> ... WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries); What stops swapoff to succeed after process 2 reclaimed the swap cache but before process 1 finished its call to swap_page_trans_huge_swapped()? > > That just leaves shmem... I suspected there might be some serialization between > shmem_unuse() (called from try_to_unuse()) and the shmem free_swap_and_cache() > callsites, but I can't see it. Hmm... > >> >> Would performing the overall operation under lock_cluster_or_swap_info help? Not >> so sure :( > > No - that function relies on being able to access the cluster from the array in > the swap_info and lock it. And I think that array has the same lifetime as > swap_map, so same problem. You'd need get_swap_device()/put_swap_device() and a > bunch of refactoring for the internals not to take the locks, I guess. I think > its doable, just not sure if neccessary... Agreed. -- Cheers, David / dhildenb