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[91.145.109.188]) by smtp.gmail.com with ESMTPSA id g3-20020a2ea4a3000000b0025e5fd96bf6sm1284615ljm.15.2022.08.14.21.06.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 14 Aug 2022 21:06:26 -0700 (PDT) Message-ID: <490e670d-40fe-6284-04d9-9be8fd15d49f@redhat.com> Date: Mon, 15 Aug 2022 07:06:25 +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 From: =?UTF-8?Q?Mika_Penttil=c3=a4?= 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> <2aa2013a-735d-a96a-2f35-0a44a06d85f0@redhat.com> In-Reply-To: <2aa2013a-735d-a96a-2f35-0a44a06d85f0@redhat.com> 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 7.05, Mika Penttilä wrote: > > > 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... Meant we skip swap cache pages also, 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 >>>> >>