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[2003:d8:2f27:ec00:4f4d:d38:ba97:9aa2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-451f9813e5dsm17901785e9.12.2025.06.05.02.13.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Jun 2025 02:13:41 -0700 (PDT) Message-ID: <9eac8a3f-08c2-41f5-a468-1fe5c00a046c@redhat.com> Date: Thu, 5 Jun 2025 11:13:39 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/1] mm: gup: avoid CMA page pinning by retrying migration if no migratable page To: Hyesoo Yu , janghyuck.kim@samsung.com, zhaoyang.huang@unisoc.com, jaewon31.kim@gmail.com, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250605080436.3764686-1-hyesoo.yu@samsung.com> <20250605080436.3764686-2-hyesoo.yu@samsung.com> <20250605083916.GA3770753@tiffany> 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: <20250605083916.GA3770753@tiffany> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 05.06.25 10:39, Hyesoo Yu wrote: > On Thu, Jun 05, 2025 at 05:04:31PM +0900, Hyesoo Yu wrote: >> Commit 1aaf8c122918 ("mm: gup: fix infinite loop within __get_longterm_locked") >> introduced an issue where CMA pages could be pinned by longterm GUP requests. >> This occurs when unpinnable pages are detected but the movable_page_list is empty; >> the commit would return success without retrying, allowing unpinnable >> pages (such as CMA) to become pinned. >> >> CMA pages may be temporarily off the LRU due to concurrent isolation, >> for example when multiple longterm GUP requests are racing and therefore >> not appear in movable_page_list. Before commit 1aaf8c, the kernel would >> retry migration in such cases, which helped avoid accidental CMA pinning. >> >> The original intent of the commit was to support longterm GUP on non-LRU >> CMA pages in out-of-tree use cases such as pKVM. However, allowing this >> can lead to broader CMA pinning issues. >> >> To avoid this, the logic is restored to return -EAGAIN instead of success >> when no folios could be collected but unpinnable pages were found. >> This ensures that migration is retried until success, and avoids >> inadvertently pinning unpinnable pages. >> >> Fixes: 1aaf8c122918 ("mm: gup: fix infinite loop within __get_longterm_locked") >> Acked-by: David Hildenbrand >> Signed-off-by: Hyesoo Yu >> >> --- >> We have confirmed that this regression causes CMA pages to be pinned >> in our kernel 6.12-based environment. >> >> In addition to CMA allocation failures, we also observed longterm GUP failures >> when repeatedly accessing the same VMA. Specifically, the first longterm GUP >> call would pin a CMA page, and a second call on the same region >> would fail the migration because the cma page was already pinned. >> >> After reverting commit 1aaf8c122918, the issue no longer reproduced. >> >> Therefore, this fix is important to ensure reliable behavior of longterm GUP >> and CMA-backed memory, and should be backported to stable. >> --- >> mm/gup.c | 28 ++++++++++++++++++++++------ >> 1 file changed, 22 insertions(+), 6 deletions(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index e065a49842a8..66193421c1d4 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -2300,14 +2300,12 @@ static void pofs_unpin(struct pages_or_folios *pofs) >> unpin_user_pages(pofs->pages, pofs->nr_entries); >> } >> >> -/* >> - * Returns the number of collected folios. Return value is always >= 0. >> - */ >> -static void collect_longterm_unpinnable_folios( >> +static bool collect_longterm_unpinnable_folios( >> struct list_head *movable_folio_list, >> struct pages_or_folios *pofs) >> { >> struct folio *prev_folio = NULL; >> + bool any_unpinnable = false; >> bool drain_allow = true; >> unsigned long i; >> >> @@ -2321,6 +2319,8 @@ static void collect_longterm_unpinnable_folios( >> if (folio_is_longterm_pinnable(folio)) >> continue; >> >> + any_unpinnable = true; >> + >> if (folio_is_device_coherent(folio)) >> continue; >> >> @@ -2342,6 +2342,8 @@ static void collect_longterm_unpinnable_folios( >> NR_ISOLATED_ANON + folio_is_file_lru(folio), >> folio_nr_pages(folio)); >> } >> + >> + return any_unpinnable; >> } >> >> /* >> @@ -2417,11 +2419,25 @@ migrate_longterm_unpinnable_folios(struct list_head *movable_folio_list, >> static long >> check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs) >> { >> + bool any_unpinnable; >> + >> LIST_HEAD(movable_folio_list); >> >> - collect_longterm_unpinnable_folios(&movable_folio_list, pofs); >> - if (list_empty(&movable_folio_list)) >> + any_unpinnable = collect_longterm_unpinnable_folios(&movable_folio_list, pofs); >> + > > Hi David, > > While re-reviewing the v3 patch, I realized that migrate_longterm_unpinnable_folios() > should always be called whenever unpinnable folios are present, regardless of whether > the movable_folio_list is empty. > > In collect_longterm_unpinnable_folios(), if folio_is_device_coherent() is true, > the folio is not added to movable_folio_list. However, such device-coherent folios > still need to be migrated later in migrate_longterm_unpinnable_folios(). Ohh, because we cannot isolate them ... and they are always longterm-unpinnable. > > I think the condition `list_empty(&movable_folio_list)` should be removed, > and it might be better to revert commit 1aaf8c122918 rather than adding a new patch. > > What do you think? Yeah, with that in mind, a revert might indeed be the better option. -- Cheers, David / dhildenb