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[2003:cb:c709:6200:16ba:af70:999d:6a1a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38bf327dd34sm12414404f8f.83.2025.01.21.00.02.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Jan 2025 00:02:44 -0800 (PST) Message-ID: <7d9d71f1-6648-41ed-a9a8-646080ca692b@redhat.com> Date: Tue, 21 Jan 2025 09:02:42 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 0/2] How HugeTLB handle HWPoison page at truncation To: Jiaqi Yan Cc: nao.horiguchi@gmail.com, linmiaohe@huawei.com, sidhartha.kumar@oracle.com, muchun.song@linux.dev, jane.chu@oracle.com, akpm@linux-foundation.org, osalvador@suse.de, rientjes@google.com, jthoughton@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250119180608.2132296-1-jiaqiyan@google.com> <6f97f3b6-3e7b-4ff8-8d67-ef972791cccd@redhat.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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: MdvpxAfjX4d0FCCIDsmt1nX66bFi_F6nmkYNDashLnA_1737446566 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: 2E7B740002 X-Stat-Signature: mmrrgqoyft5x6bfyqe4qzp6dyd6tfr34 X-Rspamd-Server: rspam08 X-Rspam-User: X-HE-Tag: 1737446570-603031 X-HE-Meta: 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 Ze2Z5mNM 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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 21.01.25 02:21, Jiaqi Yan wrote: > On Mon, Jan 20, 2025 at 2:59 AM David Hildenbrand wrote: >> >> On 19.01.25 19:06, Jiaqi Yan wrote: >>> While I was working on userspace MFR via memfd [1], I spend some time to >>> understand what current kernel does when a HugeTLB-backing memfd is >>> truncated. My expectation is, if there is a HWPoison HugeTLB folio >>> mapped via the memfd to userspace, it will be unmapped right away but >>> still be kept in page cache [2]; however when the memfd is truncated to >>> zero or after the memfd is closed, kernel should dissolve the HWPoison >>> folio in the page cache, and free only the clean raw pages to buddy >>> allocator, excluding the poisoned raw page. >>> >>> So I wrote a hugetlb-mfr-base.c selftest and expect >>> 0. say nr_hugepages initially is 64 as system configuration. >>> 1. after MADV_HWPOISON, nr_hugepages should still be 64 as we kept even >>> HWPoison huge folio in page cache. free_hugepages should be >>> nr_hugepages minus whatever the amount in use. >>> 2. after truncated memfd to zero, nr_hugepages should reduced to 63 as >>> kernel dissolved and freed the HWPoison huge folio. free_hugepages >>> should also be 63. >>> >>> However, when testing at the head of mm-stable commit 2877a83e4a0a >>> ("mm/hugetlb: use folio->lru int demote_free_hugetlb_folios()"), I found >>> although free_hugepages is reduced to 63, nr_hugepages is not reduced >>> and stay at 64. >>> >>> Is my expectation outdated? Or is this some kind of bug? >>> >>> I assume this is a bug and then digged a little bit more. It seems there >>> are two issues, or two things I don't really understand. >>> >>> 1. During try_memory_failure_hugetlb, we should increased the target >>> in-use folio's refcount via get_hwpoison_hugetlb_folio. However, >>> until the end of try_memory_failure_hugetlb, this refcout is not put. >>> I can make sense of this given we keep in-use huge folio in page >>> cache. >> >> Isn't the general rule that hwpoisoned folios have a raised refcount >> such that they won't get freed + reused? At least that's how the buddy >> deals with them, and I suspect also hugetlb? > > Thanks, David. > > I see, so it is expected that the _entire_ huge folio will always have > at least a refcount of 1, even when the folio can become "free". > > For *free* huge folio, try_memory_failure_hugetlb dissolves it and > frees the clean pages (a lot) to the buddy allocator. This made me > think the same thing will happen for *in-use* huge folio _eventually_ > (i.e. somehow the refcount due to HWPoison can be put). I feel this is > a little bit unfortunate for the clean pages, but if it is what it is, > that's fair as it is not a bug. Yes, that's my understanding. Free pages are a lot easier to handle because we can just reliably dissolve and free them. For in-unse, it's a lot more tricky. Similar to ordinary free buddy vs. allocated pages. > >> >>> [ 1069.320976] page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x2780000 >>> [ 1069.320978] head: order:18 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0 >>> [ 1069.320980] flags: 0x400000000100044(referenced|head|hwpoison|node=0|zone=1) >>> [ 1069.320982] page_type: f4(hugetlb) >>> [ 1069.320984] raw: 0400000000100044 ffffffff8760bbc8 ffffffff8760bbc8 0000000000000000 >>> [ 1069.320985] raw: 0000000000000000 0000000000000000 00000001f4000000 0000000000000000 >>> [ 1069.320987] head: 0400000000100044 ffffffff8760bbc8 ffffffff8760bbc8 0000000000000000 >>> [ 1069.320988] head: 0000000000000000 0000000000000000 00000001f4000000 0000000000000000 >>> [ 1069.320990] head: 0400000000000012 ffffdd53de000001 ffffffffffffffff 0000000000000000 >>> [ 1069.320991] head: 0000000000040000 0000000000000000 00000000ffffffff 0000000000000000 >>> [ 1069.320992] page dumped because: track hwpoison folio's ref >>> >>> 2. Even if folio's refcount do drop to zero and we get into >>> free_huge_folio, it is not clear to me which part of free_huge_folio >>> is handling the case that folio is HWPoison. In my test what I >>> observed is that evantually the folio is enqueue_hugetlb_folio()-ed. >> >> How would we get a refcount of 0 if we assume the raised refcount on a >> hwpoisoned hugetlb folio? >> >> I'm probably missing something: are you saying that you can trigger a >> hwpoisoned hugetlb folio to get reallocated again, in upstream code? > > No, I think it is just my misunderstanding. From what you said, the > expectation of HWPoison hugetlb folio is just it won't get reallocated > again, which is true. Right. > > My (wrong) expectation is, in addition to the "won't reallocated > again" part, some (large) portion of the huge folio will be freed to > the buddy allocator. On the other hand, is it something worth having / > improving? (1G - some_single_digit * 4KB) seems to be valuable to the > system, though they are all 4K. #1 and #2 above are then what needs to > be done if the improvement is worth chasing. I think one challenge is making sure that the page won't accidentally get reallocated again -- and in contrast to free hugetlb pages we cannot handle this split synchronously. I recall that we might not remember the exact page part of a hugetlb folio that was poisoned (HVO optimization), but maybe we changed that in the meantime, Likely it would be worth having, but probably not very easy to implement. -- Cheers, David / dhildenb