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[2003:cb:c71e:bf00:eba1:3ab9:ab0f:d676]) by smtp.gmail.com with ESMTPSA id k6-20020a05600c478600b004185be4baefsm4614042wmo.0.2024.05.01.23.56.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 01 May 2024 23:56:29 -0700 (PDT) Message-ID: <92289167-5655-4c51-8dfc-df7ae53fdb7b@redhat.com> Date: Thu, 2 May 2024 08:56:28 +0200 Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches To: Alistair Popple , Jason Gunthorpe Cc: Christoph Hellwig , John Hubbard , Andrew Morton , LKML , linux-rdma@vger.kernel.org, linux-mm@kvack.org, Mike Marciniszyn , Leon Romanovsky , Artemy Kovalyov , Michael Guralnik , Pak Markthub References: <20240501003117.257735-1-jhubbard@nvidia.com> <20240501121032.GA941030@nvidia.com> <87r0el3tfi.fsf@nvdebian.thelocal> Content-Language: en-US From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 02.05.24 03:05, Alistair Popple wrote: > > Jason Gunthorpe writes: > >> On Tue, Apr 30, 2024 at 10:10:43PM -0700, Christoph Hellwig wrote: >>>> + pinned = -ENOMEM; >>>> + int attempts = 0; >>>> + /* >>>> + * pin_user_pages_fast() can return -EAGAIN, due to falling back >>>> + * to gup-slow and then failing to migrate pages out of >>>> + * ZONE_MOVABLE due to a transient elevated page refcount. >>>> + * >>>> + * One retry is enough to avoid this problem, so far, but let's >>>> + * use a slightly higher retry count just in case even larger >>>> + * systems have a longer-lasting transient refcount problem. >>>> + * >>>> + */ >>>> + static const int MAX_ATTEMPTS = 3; >>>> + >>>> + while (pinned == -EAGAIN && attempts < MAX_ATTEMPTS) { >>>> + pinned = pin_user_pages_fast(cur_base, >>>> + min_t(unsigned long, >>>> + npages, PAGE_SIZE / >>>> + sizeof(struct page *)), >>>> + gup_flags, page_list); >>>> ret = pinned; >>>> - goto umem_release; >>>> + attempts++; >>>> + >>>> + if (pinned == -EAGAIN) >>>> + continue; >>>> } >>>> + if (pinned < 0) >>>> + goto umem_release; >>> >>> This doesn't make sense. IFF a blind retry is all that is needed it >>> should be done in the core functionality. I fear it's not that easy, >>> though. >> >> +1 >> >> This migration retry weirdness is a GUP issue, it needs to be solved >> in the mm not exposed to every pin_user_pages caller. >> >> If it turns out ZONE_MOVEABLE pages can't actually be reliably moved >> then it is pretty broken.. > > I wonder if we should remove the arbitrary retry limit in > migrate_pages() entirely for ZONE_MOVEABLE pages and just loop until > they migrate? By definition there should only be transient references on > these pages so why do we need to limit the number of retries in the > first place? There are some weird things that still needs fixing: vmsplice() is the example that doesn't use FOLL_LONGTERM. -- Cheers, David / dhildenb