Re: [PATCH 2/2] mm/selftests: Don't prefault in gup_longterm tests

[David Hildenbrand]

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@xxxxxxxxxx>
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > ---
> > >    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