linux-kernel - Re: [PATCH v3 2/3] selftests: soft-dirty: Add test for mprotect

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Date:   Fri, 22 Jul 2022 10:37:48 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v3 2/3] selftests: soft-dirty: Add test for mprotect

On Fri, Jul 22, 2022 at 04:07:47PM +0200, David Hildenbrand wrote:
> On 22.07.22 16:00, David Hildenbrand wrote:
> > On 22.07.22 15:44, Peter Xu wrote:
> >> On Fri, Jul 22, 2022 at 09:17:34AM +0200, David Hildenbrand wrote:
> >>> On 21.07.22 20:33, Peter Xu wrote:
> >>>> Add two soft-diryt test cases for mprotect() on both anon or file.
> >>>
> >>> s/soft-diryt/soft-dirty/
> >>
> >> Fixed.
> >>
> >>>
> >>>>
> >>>> Signed-off-by: Peter Xu <peterx@...hat.com>
> >>>> ---
> >>>>  tools/testing/selftests/vm/soft-dirty.c | 69 ++++++++++++++++++++++++-
> >>>>  1 file changed, 68 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/tools/testing/selftests/vm/soft-dirty.c b/tools/testing/selftests/vm/soft-dirty.c
> >>>> index 08ab62a4a9d0..7d93906aa43f 100644
> >>>> --- a/tools/testing/selftests/vm/soft-dirty.c
> >>>> +++ b/tools/testing/selftests/vm/soft-dirty.c
> >>>> @@ -121,13 +121,78 @@ static void test_hugepage(int pagemap_fd, int pagesize)
> >>>>  	free(map);
> >>>>  }
> >>>>  
> >>>> +static void test_mprotect(int pagemap_fd, int pagesize, bool anon)
> >>>> +{
> >>>> +	const char *type[] = {"file", "anon"};
> >>>> +	const char *fname = "./soft-dirty-test-file";
> >>>> +	int test_fd;
> >>>> +	char *map;
> >>>
> >>> Instead of fname, unlink, open, close, unlink  you can use a tmpfile
> >>>
> >>> FILE *file;
> >>>
> >>> file = tmpfile();
> >>> if (!file) {
> >>> 	ksft_test_result_fail("tmpfile() failed\n");
> >>> 	return;
> >>> }
> >>> test_fd = fileno(file);
> >>
> >> Note that tmpfile() should by default fetch from /tmp which is very
> >> possibly a tmpfs afaict.  It's tricky in this special test case since I
> >> don't think tmpfs can trigger this bug (shmem doesn't define page_mkwrite).
> >>
> > 
> > I don't think we need that? SOFTDIRTY tracking enabled should be
> > sufficient, or what am I missing?
> > 
> 
> I think you're right that it doesn't work with tmpfile. I do wonder why,
> because I'd have thought that it's sufficient for
> vma_wants_writenotify() to return "1" due to the
> vma_soft_dirty_enabled() check.

I can't say I know the whole rational of this, but I think it's below that
will start to return 0 already before the soft-dirty check:

	if (pgprot_val(vm_page_prot) !=
	    pgprot_val(vm_pgprot_modify(vm_page_prot, vm_flags)))
		return 0;

in vma_wants_writenotify().

-- 
Peter Xu