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

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Message-ID: <87lerqw72n.fsf@nvdebian.thelocal>
Date:   Mon, 15 Aug 2022 12:35:48 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Mika Penttilä <mpenttil@...hat.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, linuxram@...ibm.com,
        paulus@...abs.org
Subject: Re: [PATCH 2/2] selftests/hmm-tests: Add test for dirty bits


Mika Penttilä <mpenttil@...hat.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.

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