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

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Message-ID: <58be2b37-0c3a-06d8-35f5-50bf4b765fb2@redhat.com>
Date:   Mon, 15 Aug 2022 06:11:26 +0300
From:   Mika Penttilä <mpenttil@...hat.com>
To:     Alistair Popple <apopple@...dia.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



On 15.8.2022 5.35, Alistair Popple wrote:
> 
> 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.
> 

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
>