lkml.org 
[lkml]   [2024]   [Apr]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH 2/2] mm/selftests: Don't prefault in gup_longterm tests
From
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@redhat.com>
>>> Signed-off-by: Peter Xu <peterx@redhat.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


\
 
 \ /
  Last update: 2024-05-27 18:08    [W:0.090 / U:0.120 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site