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[91.145.109.188]) by smtp.gmail.com with ESMTPSA id h5-20020a05651c124500b0025e2cb58c6esm1271348ljh.37.2022.08.14.20.11.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 14 Aug 2022 20:11:27 -0700 (PDT) Message-ID: <58be2b37-0c3a-06d8-35f5-50bf4b765fb2@redhat.com> Date: Mon, 15 Aug 2022 06:11:26 +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> From: =?UTF-8?Q?Mika_Penttil=c3=a4?= In-Reply-To: <87lerqw72n.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 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? > 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 >