Re: [PATCH 2/2] selftests/hmm-tests: Add test for dirty bits

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On 15.8.2022 5.35, Alistair Popple wrote:

Mika Penttilä <mpenttil@xxxxxxxxxx> 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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