Messages in this thread | | | Date | Mon, 15 Aug 2022 06:11:26 +0300 | Subject | Re: [PATCH 2/2] selftests/hmm-tests: Add test for dirty bits | From | Mika Penttilä <> |
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On 15.8.2022 5.35, Alistair Popple wrote: > > Mika Penttilä <mpenttil@redhat.com> 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 >
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