linux-kernel - Re: [PATCH] hmm-tests: Fix migrate_dirty

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Date:   Tue, 13 Sep 2022 18:20:45 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        huang ying <huang.ying.caritas@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.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>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] hmm-tests: Fix migrate_dirty_page test


David Hildenbrand <david@...hat.com> writes:

> On 13.09.22 07:22, Alistair Popple wrote:
>> As noted by John Hubbard the original test relied on side effects of the
>> implementation of migrate_vma_setup() to detect if pages had been
>> swapped to disk or not. This is subject to change in future so
>> explicitly check for swap entries via pagemap instead. Fix a spelling
>> mistake while we're at it.
>> Signed-off-by: Alistair Popple <apopple@...dia.com>
>> Fixes: 5cc88e844e87 ("selftests/hmm-tests: add test for dirty bits")
>> ---
>>   tools/testing/selftests/vm/hmm-tests.c | 50 +++++++++++++++++++++++---
>>   1 file changed, 46 insertions(+), 4 deletions(-)
>> diff --git a/tools/testing/selftests/vm/hmm-tests.c
>> b/tools/testing/selftests/vm/hmm-tests.c
>> index 70fdb49b59ed..b5f6a7dc1f12 100644
>> --- a/tools/testing/selftests/vm/hmm-tests.c
>> +++ b/tools/testing/selftests/vm/hmm-tests.c
>> @@ -1261,9 +1261,47 @@ static int destroy_cgroup(void)
>>   	return 0;
>>   }
>>   +static uint64_t get_pfn(int fd, uint64_t ptr)
>> +{
>> +	uint64_t pfn;
>> +	int ret;
>> +
>> +	ret = pread(fd, &pfn, sizeof(ptr),
>> +		(uint64_t) ptr / getpagesize() * sizeof(ptr));
>> +	if (ret != sizeof(ptr))
>> +		return 0;
>> +
>> +	return pfn;
>> +}
>> +
>> +#define PAGEMAP_SWAPPED (1ULL << 62)
>> +
>> +/* Returns true if at least one page in the range is on swap */
>> +static bool pages_swapped(void *ptr, unsigned long pages)
>> +{
>> +	uint64_t pfn;
>> +	int fd = open("/proc/self/pagemap", O_RDONLY);
>> +	unsigned long i;
>> +
>> +	if (fd < 0)
>> +		return false;
>> +
>> +	for (i = 0; i < pages; i++) {
>> +		pfn = get_pfn(fd, (uint64_t) ptr + i * getpagesize());
>> +
>> +		if (pfn & PAGEMAP_SWAPPED) {
>> +			close(fd);
>> +			return true;
>> +		}
>
> We do have pagemap_get_entry() in vm_util.c to query the pagemap entry.

Thanks. I'd missed that, although `grep pagemap
tools/testing/selftests/vm` suggests I'm not the first to follow a
tradition of open-coding this :-)

But there's no need to perpetuate that tradition, so will redo this to
use vm_util.c instead.

> Can you further, add pagemap_is_swapped() to vm_util.c?
>
> I'll be also needing that (including a variant for testing a range) in anon COW
> tests.