Messages in this thread | | | Date | Mon, 29 Apr 2024 15:26:22 +0200 | Subject | Re: [PATCH 2/2] mm/selftests: Don't prefault in gup_longterm tests | From | David Hildenbrand <> |
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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
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