linux-kernel - Re: [PATCH 2/2] mm/selftests: Don't prefault in gup

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Date: Mon, 29 Apr 2024 15:26:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
 Lorenzo Stoakes <lstoakes@...il.com>, John Hubbard <jhubbard@...dia.com>,
 linuxppc-dev@...ts.ozlabs.org, Muchun Song <muchun.song@...ux.dev>,
 Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH 2/2] mm/selftests: Don't prefault in gup_longterm tests

On 29.04.24 15:10, Peter Xu wrote:
> On Mon, Apr 29, 2024 at 09:28:15AM +0200, David Hildenbrand wrote:
>> On 28.04.24 21:01, Peter Xu wrote:
>>> Prefault, especially with RW, makes the GUP test too easy, and may not yet
>>> reach the core of the test.
>>>
>>> For example, R/O longterm pins will just hit, pte_write()==true for
>>> whatever cases, the unsharing logic won't be ever tested.
>>>
>>> This patch remove the prefault.  This tortures more code paths at least to
>>> cover the unshare care for R/O longterm pins, in which case the first R/O
>>> GUP attempt will fault in the page R/O first, then the 2nd will go through
>>> the unshare path, checking whether an unshare is needed.
>>>
>>> Cc: David Hildenbrand <david@...hat.com>
>>> Signed-off-by: Peter Xu <peterx@...hat.com>
>>> ---
>>>    tools/testing/selftests/mm/gup_longterm.c | 12 +++++++++---
>>>    1 file changed, 9 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/tools/testing/selftests/mm/gup_longterm.c b/tools/testing/selftests/mm/gup_longterm.c
>>> index ad168d35b23b..488e32186246 100644
>>> --- a/tools/testing/selftests/mm/gup_longterm.c
>>> +++ b/tools/testing/selftests/mm/gup_longterm.c
>>> @@ -119,10 +119,16 @@ static void do_test(int fd, size_t size, enum test_type type, bool shared)
>>>    	}
>>>    	/*
>>> -	 * Fault in the page writable such that GUP-fast can eventually pin
>>> -	 * it immediately.
>>> +	 * Explicitly avoid pre-faulting in the page, this can help testing
>>> +	 * more code paths.
>>> +	 *
>>> +	 * Take example of an upcoming R/O pin test, if we RW prefault the
>>> +	 * page, such pin will directly skip R/O unsharing and the longterm
>>> +	 * pin will success mostly always.  When not prefaulted, R/O
>>> +	 * longterm pin will first fault in a RO page, then the 2nd round
>>> +	 * it'll go via the unshare check.  Otherwise those paths aren't
>>> +	 * covered.
>>>    	 */
>> This will mean that GUP-fast never succeeds, which removes quite some testing
>> coverage for most other tests here.
>>
>> Note that the main motivation of this test was to test gup_fast_folio_allowed(),
>> where we had issues with GUP-fast during development.
> 
> Ah I didn't notice that, as I thought that whitelists memfd ones.
> 
>>
>> Would the following also get the job done?
>>
>> diff --git a/tools/testing/selftests/mm/gup_longterm.c b/tools/testing/selftests/mm/gup_longterm.c
>> index ad168d35b23b7..e917a7c58d571 100644
>> --- a/tools/testing/selftests/mm/gup_longterm.c
>> +++ b/tools/testing/selftests/mm/gup_longterm.c
>> @@ -92,7 +92,7 @@ static void do_test(int fd, size_t size, enum test_type type, bool shared)
>>   {
>>   	__fsword_t fs_type = get_fs_type(fd);
>>   	bool should_work;
>> -	char *mem;
>> +	char tmp, *mem;
>>   	int ret;
>>   	if (ftruncate(fd, size)) {
>> @@ -119,10 +119,19 @@ static void do_test(int fd, size_t size, enum test_type type, bool shared)
>>   	}
>>   	/*
>> -	 * Fault in the page writable such that GUP-fast can eventually pin
>> -	 * it immediately.
>> +	 * Fault in the page such that GUP-fast might be able to pin it
>> +	 * immediately. To cover more cases, don't fault in pages writable when
>> +	 * R/O pinning.
>>   	 */
>> -	memset(mem, 0, size);
>> +	switch (type) {
>> +	case TEST_TYPE_RO:
>> +	case TEST_TYPE_RO_FAST:
>> +		tmp = *mem;
>> +		asm volatile("" : "+r" (tmp));
>> +		break;
>> +	default:
>> +		memset(mem, 0, size);
>> +	};
>>   	switch (type) {
>>   	case TEST_TYPE_RO:
> 
> Yes this could work too.
> 
> The test patch here doesn't need to rush. David, how about you prepare a
> better and verified patch and post it separately, making sure to cover all
> the things we used to cover plus the unshare?  IIUC it used to be not
> touched because of pte_write() always returns true with a write prefault.
> 
> Then we let patch 1 go through first, and drop this one?

Whatever you prefer!

-- 
Cheers,

David / dhildenb