linux-kernel - Re: [PATCH v3 6/7] selftest/mm: ksm_functional_tests: test in mmap_and_merge

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Message-ID: <859496c5-3971-0b65-c297-d29083719a75@redhat.com>
Date:   Fri, 4 Aug 2023 19:56:28 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        liubo <liubo254@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Mel Gorman <mgorman@...e.de>, Shuah Khan <shuah@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 6/7] selftest/mm: ksm_functional_tests: test in
 mmap_and_merge_range() if anything got merged

On 03.08.23 21:05, Peter Xu wrote:
> On Thu, Aug 03, 2023 at 04:32:07PM +0200, David Hildenbrand wrote:
>> Let's extend mmap_and_merge_range() to test if anything in the current
>> process was merged. range_maps_duplicates() is too unreliable for that
>> use case, so instead look at KSM stats.
>>
>> Trigger a complete unmerge first, to cleanup the stable tree and
>> stabilize accounting of merged pages.
>>
>> Note that we're using /proc/self/ksm_merging_pages instead of
>> /proc/self/ksm_stat, because that one is available in more existing
>> kernels.
>>
>> If /proc/self/ksm_merging_pages can't be opened, we can't perform any
>> checks and simply skip them.
>>
>> We have to special-case the shared zeropage for now. But the only user
>> -- test_unmerge_zero_pages() -- performs its own merge checks.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 

Hi Peter,

thanks for the review!

> Acked-by: Peter Xu <peterx@...hat.com>
> 
> One nitpick:
> 
>> ---
>>   .../selftests/mm/ksm_functional_tests.c       | 47 +++++++++++++++++++
>>   1 file changed, 47 insertions(+)
>>
>> diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
>> index 0de9d33cd565..cb63b600cb4f 100644
>> --- a/tools/testing/selftests/mm/ksm_functional_tests.c
>> +++ b/tools/testing/selftests/mm/ksm_functional_tests.c
>> @@ -30,6 +30,7 @@
>>   static int ksm_fd;
>>   static int ksm_full_scans_fd;
>>   static int proc_self_ksm_stat_fd;
>> +static int proc_self_ksm_merging_pages_fd;
>>   static int ksm_use_zero_pages_fd;
>>   static int pagemap_fd;
>>   static size_t pagesize;
>> @@ -88,6 +89,22 @@ static long get_my_ksm_zero_pages(void)
>>   	return my_ksm_zero_pages;
>>   }
>>   
>> +static long get_my_merging_pages(void)
>> +{
>> +	char buf[10];
>> +	ssize_t ret;
>> +
>> +	if (proc_self_ksm_merging_pages_fd < 0)
>> +		return proc_self_ksm_merging_pages_fd;
> 
> Better do the fds check all in main(), e.g. not all callers below considers
> negative values, so -1 can pass "if (get_my_merging_pages())" etc.

The two existing callers should be handling it correctly:

if (get_my_merging_pages() > 0)
	-> fail

if (val && !get_my_merging_pages()
	-> fail

Both will pass on negative values, unless I am missing something.

I tried to keep the test working also on older kernels where 
ksm_merging_pages does not exist yet (it's ~1 year old).


Anyhow, if you think it's better to make the test fail on these setups, 
I can change it.

-- 
Cheers,

David / dhildenb