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[79.242.63.241]) by smtp.gmail.com with ESMTPSA id p10-20020a05600c468a00b00416ba2db6b4sm6463514wmo.18.2024.04.12.12.06.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Apr 2024 12:06:33 -0700 (PDT) Message-ID: Date: Fri, 12 Apr 2024 21:06:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list To: Zi Yan , Yang Shi Cc: linux-mm@kvack.org, Andrew Morton , Matthew Wilcox , Ryan Roberts , Barry Song <21cnbao@gmail.com>, linux-kernel@vger.kernel.org References: <20240411153232.169560-1-zi.yan@sent.com> <86722546-1d54-4224-9f31-da4f368cd47e@redhat.com> <0A4D2CA1-E156-43E9-A1C9-E09E62E760A3@nvidia.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-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 9714C140008 X-Rspam-User: X-Stat-Signature: kgc51e3az6cmsfotho93o4ceophdgwx1 X-Rspamd-Server: rspam01 X-HE-Tag: 1712948798-359951 X-HE-Meta: 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 WVENBN1e 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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 12.04.24 16:31, Zi Yan wrote: > On 12 Apr 2024, at 10:21, Zi Yan wrote: > >> On 11 Apr 2024, at 17:59, Yang Shi wrote: >> >>> On Thu, Apr 11, 2024 at 2:15 PM David Hildenbrand wrote: >>>> >>>> On 11.04.24 21:01, Yang Shi wrote: >>>>> On Thu, Apr 11, 2024 at 8:46 AM David Hildenbrand wrote: >>>>>> >>>>>> On 11.04.24 17:32, Zi Yan wrote: >>>>>>> From: Zi Yan >>>>>>> >>>>>>> In __folio_remove_rmap(), a large folio is added to deferred split list >>>>>>> if any page in a folio loses its final mapping. It is possible that >>>>>>> the folio is unmapped fully, but it is unnecessary to add the folio >>>>>>> to deferred split list at all. Fix it by checking folio mapcount before >>>>>>> adding a folio to deferred split list. >>>>>>> >>>>>>> Signed-off-by: Zi Yan >>>>>>> --- >>>>>>> mm/rmap.c | 9 ++++++--- >>>>>>> 1 file changed, 6 insertions(+), 3 deletions(-) >>>>>>> >>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>>>> index 2608c40dffad..d599a772e282 100644 >>>>>>> --- a/mm/rmap.c >>>>>>> +++ b/mm/rmap.c >>>>>>> @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>>>>>> enum rmap_level level) >>>>>>> { >>>>>>> atomic_t *mapped = &folio->_nr_pages_mapped; >>>>>>> - int last, nr = 0, nr_pmdmapped = 0; >>>>>>> + int last, nr = 0, nr_pmdmapped = 0, mapcount = 0; >>>>>>> enum node_stat_item idx; >>>>>>> >>>>>>> __folio_rmap_sanity_checks(folio, page, nr_pages, level); >>>>>>> @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>>>>>> break; >>>>>>> } >>>>>>> >>>>>>> - atomic_sub(nr_pages, &folio->_large_mapcount); >>>>>>> + mapcount = atomic_sub_return(nr_pages, >>>>>>> + &folio->_large_mapcount) + 1; >>>>>> >>>>>> That becomes a new memory barrier on some archs. Rather just re-read it >>>>>> below. Re-reading should be fine here. >>>>>> >>>>>>> do { >>>>>>> last = atomic_add_negative(-1, &page->_mapcount); >>>>>>> if (last) { >>>>>>> @@ -1554,7 +1555,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>>>>>> * is still mapped. >>>>>>> */ >>>>>>> if (folio_test_large(folio) && folio_test_anon(folio)) >>>>>>> - if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) >>>>>>> + if ((level == RMAP_LEVEL_PTE && >>>>>>> + mapcount != 0) || >>>>>>> + (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)) >>>>>>> deferred_split_folio(folio); >>>>>>> } >>>>>> >>>>>> But I do wonder if we really care? Usually the folio will simply get >>>>>> freed afterwards, where we simply remove it from the list. >>>>>> >>>>>> If it's pinned, we won't be able to free or reclaim, but it's rather a >>>>>> corner case ... >>>>>> >>>>>> Is it really worth the added code? Not convinced. >>>>> >>>>> It is actually not only an optimization, but also fixed the broken >>>>> thp_deferred_split_page counter in /proc/vmstat. >>>>> >>>>> The counter actually counted the partially unmapped huge pages (so >>>>> they are on deferred split queue), but it counts the fully unmapped >>>>> mTHP as well now. For example, when a 64K THP is fully unmapped, the >>>>> thp_deferred_split_page is not supposed to get inc'ed, but it does >>>>> now. >>>>> >>>>> The counter is also useful for performance analysis, for example, >>>>> whether a workload did a lot of partial unmap or not. So fixing the >>>>> counter seems worthy. Zi Yan should have mentioned this in the commit >>>>> log. >>>> >>>> Yes, all that is information that is missing from the patch description. >>>> If it's a fix, there should be a "Fixes:". >>>> >>>> Likely we want to have a folio_large_mapcount() check in the code below. >>>> (I yet have to digest the condition where this happens -- can we have an >>>> example where we'd use to do the wrong thing and now would do the right >>>> thing as well?) >>> >>> For example, map 1G memory with 64K mTHP, then unmap the whole 1G or >>> some full 64K areas, you will see thp_deferred_split_page increased, >>> but it shouldn't. >>> >>> It looks __folio_remove_rmap() incorrectly detected whether the mTHP >>> is fully unmapped or partially unmapped by comparing the number of >>> still-mapped subpages to ENTIRELY_MAPPED, which should just work for >>> PMD-mappable THP. >>> >>> However I just realized this problem was kind of workaround'ed by commit: >>> >>> commit 98046944a1597f3a02b792dbe9665e9943b77f28 >>> Author: Baolin Wang >>> Date: Fri Mar 29 14:59:33 2024 +0800 >>> >>> mm: huge_memory: add the missing folio_test_pmd_mappable() for THP >>> split statistics >>> >>> Now the mTHP can also be split or added into the deferred list, so add >>> folio_test_pmd_mappable() validation for PMD mapped THP, to avoid >>> confusion with PMD mapped THP related statistics. >>> >>> Link: https://lkml.kernel.org/r/a5341defeef27c9ac7b85c97f030f93e4368bbc1.1711694852.git.baolin.wang@linux.alibaba.com >>> Signed-off-by: Baolin Wang >>> Acked-by: David Hildenbrand >>> Cc: Muchun Song >>> Signed-off-by: Andrew Morton >>> >>> This commit made thp_deferred_split_page didn't count mTHP anymore, it >>> also made thp_split_page didn't count mTHP anymore. >>> >>> However Zi Yan's patch does make the code more robust and we don't >>> need to worry about the miscounting issue anymore if we will add >>> deferred_split_page and split_page counters for mTHP in the future. >> >> Actually, the patch above does not fix everything. A fully unmapped >> PTE-mapped order-9 THP is also added to deferred split list and >> counted as THP_DEFERRED_SPLIT_PAGE without my patch, since nr is 512 >> (non zero), level is RMAP_LEVEL_PTE, and inside deferred_split_folio() >> the order-9 folio is folio_test_pmd_mappable(). >> >> I will add this information in the next version. > > It might > Fixes: b06dc281aa99 ("mm/rmap: introduce folio_remove_rmap_[pte|ptes|pmd]()"), > but before this commit fully unmapping a PTE-mapped order-9 THP still increased > THP_DEFERRED_SPLIT_PAGE, because PTEs are unmapped individually and first PTE > unmapping adds the THP into the deferred split list. This means commit b06dc281aa99 > did not change anything and before that THP_DEFERRED_SPLIT_PAGE increase is > due to implementation. I will add this to the commit log as well without Fixes > tag. Right, so it's always been a problem for PTE-mapped PMD-sized THP and only with batching we can now do "better". But not fix it completely. I'll reply separately to your other mail. -- Cheers, David / dhildenb