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[2003:d8:2f0d:f000:eec9:2b8d:4913:f32a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-450d7fa25besm159610045e9.14.2025.06.03.06.15.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Jun 2025 06:15:04 -0700 (PDT) Message-ID: Date: Tue, 3 Jun 2025 15:15:03 +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 v4 4/5] kdump: wait for DMA to finish when using CMA To: Jiri Bohac , Baoquan He , Vivek Goyal , Dave Young , kexec@lists.infradead.org Cc: Philipp Rudo , Donald Dutile , Pingfan Liu , Tao Liu , linux-kernel@vger.kernel.org, David Hildenbrand , Michal Hocko References: 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.05.25 22:29, Jiri Bohac wrote: > When re-using the CMA area for kdump there is a risk of pending DMA into > pinned user pages in the CMA area. > > Pages that are pinned long-term are migrated away from CMA, so these are > not a concern. Pages pinned without FOLL_LONGTERM remain in the CMA and may > possibly be the source or destination of a pending DMA transfer. I'll note that we right now do have an upstream BUG where that is sometimes not the case. I mentioned it previously that such bugs will be a problem :( https://lkml.kernel.org/r/20250523023709epcms1p236d4f55b79adb9366ec1cf6d5792b06b@epcms1p2 > > Although there is no clear specification how long a page may be pinned > without FOLL_LONGTERM, pinning without the flag shows an intent of the > caller to only use the memory for short-lived DMA transfers, not a transfer > initiated by a device asynchronously at a random time in the future. > > Add a delay of CMA_DMA_TIMEOUT_SEC seconds before starting the kdump > kernel, giving such short-lived DMA transfers time to finish before the CMA > memory is re-used by the kdump kernel. > > Set CMA_DMA_TIMEOUT_SEC to 10 seconds - chosen arbitrarily as both > a huge margin for a DMA transfer, yet not increasing the kdump time > too significantly. > > Signed-off-by: Jiri Bohac > > --- > Changes since v3: > - renamed CMA_DMA_TIMEOUT_SEC to CMA_DMA_TIMEOUT_MSEC, change delay to 10 seconds > - introduce a cma_dma_timeout_sec initialized to CMA_DMA_TIMEOUT_SEC > to make the timeout trivially tunable if needed in the future > > --- > include/linux/crash_core.h | 3 +++ > kernel/crash_core.c | 17 +++++++++++++++++ > 2 files changed, 20 insertions(+) > > diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h > index 44305336314e..805a07042c96 100644 > --- a/include/linux/crash_core.h > +++ b/include/linux/crash_core.h > @@ -56,6 +56,9 @@ static inline unsigned int crash_get_elfcorehdr_size(void) { return 0; } > /* Alignment required for elf header segment */ > #define ELF_CORE_HEADER_ALIGN 4096 > > +/* Default value for cma_dma_timeout_sec */ > +#define CMA_DMA_TIMEOUT_SEC 10 > + > extern int crash_exclude_mem_range(struct crash_mem *mem, > unsigned long long mstart, > unsigned long long mend); > diff --git a/kernel/crash_core.c b/kernel/crash_core.c > index 335b8425dd4b..a255c9e2ef29 100644 > --- a/kernel/crash_core.c > +++ b/kernel/crash_core.c > @@ -21,6 +21,7 @@ > #include > #include > #include > +#include > > #include > #include > @@ -33,6 +34,11 @@ > /* Per cpu memory for storing cpu states in case of system crash. */ > note_buf_t __percpu *crash_notes; > > +/* time to wait for possible DMA to finish before starting the kdump kernel > + * when a CMA reservation is used > + */ > +unsigned int cma_dma_timeout_sec = CMA_DMA_TIMEOUT_SEC; > + > #ifdef CONFIG_CRASH_DUMP > > int kimage_crash_copy_vmcoreinfo(struct kimage *image) > @@ -97,6 +103,17 @@ int kexec_crash_loaded(void) > } > EXPORT_SYMBOL_GPL(kexec_crash_loaded); > > +static void crash_cma_clear_pending_dma(void) > +{ > + unsigned int s = cma_dma_timeout_sec; > + > + if (!crashk_cma_cnt) > + return; > + > + while (s--) > + mdelay(1000); Any reason we cannot do it in a single mdelay() invocation? mdelay() already is a loop around udelay on larger values IIUC. -- Cheers, David / dhildenb