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

[Mika Penttilä]

On 15.8.2022 6.21, Alistair Popple wrote:
> Mika Penttilä <mpenttil@xxxxxxxxxx> writes:
>
> > 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?
>
> 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