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[91.145.109.188]) by smtp.gmail.com with ESMTPSA id m14-20020a19434e000000b0048aef1abb08sm957530lfj.297.2022.08.14.21.05.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 14 Aug 2022 21:05:02 -0700 (PDT) Message-ID: <2aa2013a-735d-a96a-2f35-0a44a06d85f0@redhat.com> Date: Mon, 15 Aug 2022 07:05:01 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH 2/2] selftests/hmm-tests: Add test for dirty bits Content-Language: en-US To: Alistair Popple Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, "Sierra Guiza, Alejandro (Alex)" , Felix Kuehling , Jason Gunthorpe , John Hubbard , David Hildenbrand , Ralph Campbell , Matthew Wilcox , Karol Herbst , Lyude Paul , Ben Skeggs , Logan Gunthorpe , linuxram@us.ibm.com, paulus@ozlabs.org References: <8f19b172d32be2e889b837f88b1ba070bf2c97ee.1660281458.git-series.apopple@nvidia.com> <1ec090fa-f93b-c197-e5b3-ff2b0d5862ef@redhat.com> <87lerqw72n.fsf@nvdebian.thelocal> <58be2b37-0c3a-06d8-35f5-50bf4b765fb2@redhat.com> <87h72ew4p6.fsf@nvdebian.thelocal> From: =?UTF-8?Q?Mika_Penttil=c3=a4?= In-Reply-To: <87h72ew4p6.fsf@nvdebian.thelocal> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15.8.2022 6.21, Alistair Popple wrote: > > Mika Penttilä writes: > >> On 15.8.2022 5.35, Alistair Popple wrote: >>> Mika Penttilä writes: >>> >>>> Hi Alistair! >>>> >>>> On 12.8.2022 8.22, Alistair Popple wrote: >>> [...] >>> >>>>> + buffer->ptr = mmap(NULL, size, >>>>> + PROT_READ | PROT_WRITE, >>>>> + MAP_PRIVATE | MAP_ANONYMOUS, >>>>> + buffer->fd, 0); >>>>> + ASSERT_NE(buffer->ptr, MAP_FAILED); >>>>> + >>>>> + /* Initialize buffer in system memory. */ >>>>> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i) >>>>> + ptr[i] = 0; >>>>> + >>>>> + ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30)); >>>>> + >>>>> + /* Fault pages back in from swap as clean pages */ >>>>> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i) >>>>> + tmp += ptr[i]; >>>>> + >>>>> + /* Dirty the pte */ >>>>> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i) >>>>> + ptr[i] = i; >>>>> + >>>> >>>> The anon pages are quite likely in memory at this point, and dirty in pte. >>> Why would the pte be dirty? I just confirmed using some modified pagemap >>> code that on my system at least this isn't the case. >>> >>>>> + /* >>>>> + * Attempt to migrate memory to device, which should fail because >>>>> + * hopefully some pages are backed by swap storage. >>>>> + */ >>>>> + ASSERT_TRUE(hmm_migrate_sys_to_dev(self->fd, buffer, npages)); >>>> >>>> And pages marked dirty also now. But could you elaborate how and where the above >>>> fails in more detail, couldn't immediately see it... >>> Not if you don't have patch 1 of this series applied. If the >>> trylock_page() in migrate_vma_collect_pmd() succeeds (which it almost >>> always does) it will have cleared the pte without setting PageDirty. >>> >> >> Ah yes but I meant with the patch 1 applied, the comment "Attempt to migrate >> memory to device, which should fail because hopefully some pages are backed by >> swap storage" indicates that hmm_migrate_sys_to_dev() would fail..and there's >> that ASSERT_TRUE which means fail here. >> >> So I understand the data loss but where is the hmm_migrate_sys_to_dev() failing, >> with or wihtout patch 1 applied? > > Oh right. hmm_migrate_sys_to_dev() will fail because the page is in the > swap cache, and migrate_vma_*() doesn't currently support migrating > pages with a mapping. > Ok I forgot we skip also page cache pages, not just file pages... >>> So now we have a dirty page without PageDirty set and without a dirty >>> pte. If this page gets swapped back to disk and is still in the swap >>> cache data will be lost because reclaim will see a clean page and won't >>> write it out again. >>> At least that's my understanding - please let me know if you see >>> something that doesn't make sense. >>> >>>>> + >>>>> + ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30)); >>>>> + >>>>> + /* Check we still see the updated data after restoring from swap. */ >>>>> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i) >>>>> + ASSERT_EQ(ptr[i], i); >>>>> + >>>>> + hmm_buffer_free(buffer); >>>>> + destroy_cgroup(); >>>>> +} >>>>> + >>>>> /* >>>>> * Read anonymous memory multiple times. >>>>> */ >>>> >>>> >>>> --Mika >>> >