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[79.242.49.153]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36bbcf0dc8fsm16402391f8f.6.2024.08.07.07.52.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 07 Aug 2024 07:52:33 -0700 (PDT) Message-ID: <20e91e84-1e20-4998-935a-b310e6d9be5f@redhat.com> Date: Wed, 7 Aug 2024 16:52:31 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 07/11] mm/huge_memory: convert split_huge_pages_pid() from follow_page() to folio_walk To: Zi Yan Cc: Ryan Roberts , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, kvm@vger.kernel.org, linux-s390@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Jonathan Corbet , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Sven Schnelle , Gerald Schaefer , Mark Brown References: <20240802155524.517137-1-david@redhat.com> <20240802155524.517137-8-david@redhat.com> <5BEF38E0-359C-4927-98EF-A0EE7DC81251@nvidia.com> <2D2B77E0-66BE-4ECE-8262-3E28D7D073E6@nvidia.com> From: David Hildenbrand Content-Language: en-US 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: <2D2B77E0-66BE-4ECE-8262-3E28D7D073E6@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07.08.24 16:45, Zi Yan wrote: > On 7 Aug 2024, at 5:57, David Hildenbrand wrote: > >> On 06.08.24 17:36, Zi Yan wrote: >>> On 6 Aug 2024, at 6:24, David Hildenbrand wrote: >>> >>>> On 06.08.24 12:03, David Hildenbrand wrote: >>>>> On 06.08.24 11:56, David Hildenbrand wrote: >>>>>> On 06.08.24 11:46, Ryan Roberts wrote: >>>>>>> On 02/08/2024 16:55, David Hildenbrand wrote: >>>>>>>> Let's remove yet another follow_page() user. Note that we have to do the >>>>>>>> split without holding the PTL, after folio_walk_end(). We don't care >>>>>>>> about losing the secretmem check in follow_page(). >>>>>>> >>>>>>> Hi David, >>>>>>> >>>>>>> Our (arm64) CI is showing a regression in split_huge_page_test from mm selftests from next-20240805 onwards. Navigating around a couple of other lurking bugs, I was able to bisect to this change (which smells about right). >>>>>>> >>>>>>> Newly failing test: >>>>>>> >>>>>>> # # ------------------------------ >>>>>>> # # running ./split_huge_page_test >>>>>>> # # ------------------------------ >>>>>>> # # TAP version 13 >>>>>>> # # 1..12 >>>>>>> # # Bail out! Still AnonHugePages not split >>>>>>> # # # Planned tests != run tests (12 != 0) >>>>>>> # # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0 >>>>>>> # # [FAIL] >>>>>>> # not ok 52 split_huge_page_test # exit=1 >>>>>>> >>>>>>> It's trying to split some pmd-mapped THPs then checking and finding that they are not split. The split is requested via /sys/kernel/debug/split_huge_pages, which I believe ends up in this function you are modifying here. Although I'll admit that looking at the change, there is nothing obviously wrong! Any ideas? >>>>>> >>>>>> Nothing jumps at me as well. Let me fire up the debugger :) >>>>> >>>>> Ah, very likely the can_split_folio() check expects a raised refcount >>>>> already. >>>> >>>> Indeed, the following does the trick! Thanks Ryan, I could have sworn >>>> I ran that selftest as well. >>>> >>>> TAP version 13 >>>> 1..12 >>>> ok 1 Split huge pages successful >>>> ok 2 Split PTE-mapped huge pages successful >>>> # Please enable pr_debug in split_huge_pages_in_file() for more info. >>>> # Please check dmesg for more information >>>> ok 3 File-backed THP split test done >>>> >>>> ... >>>> >>>> >>>> @Andrew, can you squash the following? >>>> >>>> >>>> From e5ea585de3e089ea89bf43d8447ff9fc9b371286 Mon Sep 17 00:00:00 2001 >>>> From: David Hildenbrand >>>> Date: Tue, 6 Aug 2024 12:08:17 +0200 >>>> Subject: [PATCH] fixup: mm/huge_memory: convert split_huge_pages_pid() from >>>> follow_page() to folio_walk >>>> >>>> We have to teach can_split_folio() that we are not holding an additional >>>> reference. >>>> >>>> Signed-off-by: David Hildenbrand >>>> --- >>>> include/linux/huge_mm.h | 4 ++-- >>>> mm/huge_memory.c | 8 ++++---- >>>> mm/vmscan.c | 2 +- >>>> 3 files changed, 7 insertions(+), 7 deletions(-) >>>> >>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h >>>> index e25d9ebfdf89..ce44caa40eed 100644 >>>> --- a/include/linux/huge_mm.h >>>> +++ b/include/linux/huge_mm.h >>>> @@ -314,7 +314,7 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add >>>> unsigned long len, unsigned long pgoff, unsigned long flags, >>>> vm_flags_t vm_flags); >>>> -bool can_split_folio(struct folio *folio, int *pextra_pins); >>>> +bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins); >>>> int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, >>>> unsigned int new_order); >>>> static inline int split_huge_page(struct page *page) >>>> @@ -470,7 +470,7 @@ thp_get_unmapped_area_vmflags(struct file *filp, unsigned long addr, >>>> } >>>> static inline bool >>>> -can_split_folio(struct folio *folio, int *pextra_pins) >>>> +can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins) >>>> { >>>> return false; >>>> } >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index 697fcf89f975..c40b0dcc205b 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -3021,7 +3021,7 @@ static void __split_huge_page(struct page *page, struct list_head *list, >>>> } >>>> /* Racy check whether the huge page can be split */ >>>> -bool can_split_folio(struct folio *folio, int *pextra_pins) >>>> +bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins) >>>> { >>>> int extra_pins; >>>> @@ -3033,7 +3033,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) >>>> extra_pins = folio_nr_pages(folio); >>>> if (pextra_pins) >>>> *pextra_pins = extra_pins; >>>> - return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - 1; >>>> + return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - caller_pins; >>>> } >>>> /* >>>> @@ -3201,7 +3201,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, >>>> * Racy check if we can split the page, before unmap_folio() will >>>> * split PMDs >>>> */ >>>> - if (!can_split_folio(folio, &extra_pins)) { >>>> + if (!can_split_folio(folio, 1, &extra_pins)) { >>>> ret = -EAGAIN; >>>> goto out_unlock; >>>> } >>>> @@ -3537,7 +3537,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start, >>>> * can be split or not. So skip the check here. >>>> */ >>>> if (!folio_test_private(folio) && >>>> - !can_split_folio(folio, NULL)) >>>> + !can_split_folio(folio, 0, NULL)) >>>> goto next; >>>> if (!folio_trylock(folio)) >>> >>> The diff below can skip a folio with private and extra pin(s) early instead >>> of trying to lock and split it then failing at can_split_folio() inside >>> split_huge_page_to_list_to_order(). >>> >>> Maybe worth applying on top of yours? >>> >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index a218320a9233..ce992d54f1da 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -3532,13 +3532,10 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start, >>> goto next; >>> >>> total++; >>> - /* >>> - * For folios with private, split_huge_page_to_list_to_order() >>> - * will try to drop it before split and then check if the folio >>> - * can be split or not. So skip the check here. >>> - */ >>> - if (!folio_test_private(folio) && >>> - !can_split_folio(folio, 0, NULL)) >>> + >>> + if (!can_split_folio(folio, >>> + folio_test_private(folio) ? 1 : 0, >>> + NULL)) >> >> Hmm, it does look a bit odd. It's not something from the caller (caller_pins), but a >> folio property. Likely should be handled differently. >> >> In vmscan code, we only call can_split_folio() on anon folios where >> folio_test_private() does not apply. >> >> But indeed, in split_huge_page_to_list_to_order() we'd have to fail if >> folio_test_private() still applies after >> >> Not sure if that is really better: > > Yeah, not worth the code churn to optimize for that debugfs code. > > As I looked at this patch and the fix long enough, feel free to add > Reviewed-by: Zi Yan Thanks! :) -- Cheers, David / dhildenb