Re: [PATCH v2 2/2] selftests/mm/hmm-tests: test pagemap reads of PMD device-private entries

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, Jun 01, 2026 at 10:40:48AM +0530, Dev Jain wrote:
>
>
> On 01/06/26 5:39 am, Balbir Singh wrote:
> > On 5/30/26 18:54, Dev Jain wrote:
> >> To cover pagemap paths scanning PMD entries, add assertions to check
> >> whether a device-private PMD entry has the correct pagemap information -
> >> the PM_SWAP bit must be on in the pagemap entry. Before that, we must
> >> assert through HMM_DMIRROR_SNAPSHOT snapshot that the leaf entry is
> >> at PMD level and not PTE level.
> >>
> >> Signed-off-by: Dev Jain <dev.jain@arm.com>
> >> ---
> >>  tools/testing/selftests/mm/hmm-tests.c | 29 ++++++++++++++++++++++++++
> >>  1 file changed, 29 insertions(+)
> >>
> >> diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
> >> index e1c8a679a4cf3..d09d4a9081de1 100644
> >> --- a/tools/testing/selftests/mm/hmm-tests.c
> >> +++ b/tools/testing/selftests/mm/hmm-tests.c
> >> @@ -2276,8 +2276,11 @@ TEST_F(hmm, migrate_anon_huge_fault)
> >>  	unsigned long npages;
> >>  	unsigned long size;
> >>  	unsigned long i;
> >> +	unsigned char *m;
> >> +	uint64_t entry;
> >>  	void *old_ptr;
> >>  	void *map;
> >> +	int pagemap_fd;
> >>  	int *ptr;
> >>  	int ret;
> >>
> >> @@ -2318,6 +2321,32 @@ TEST_F(hmm, migrate_anon_huge_fault)
> >>  	for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
> >>  		ASSERT_EQ(ptr[i], i);
> >>
> >> +	if (!hmm_is_coherent_type(variant->device_number)) {
> >> +		ret = hmm_dmirror_cmd(self->fd, HMM_DMIRROR_SNAPSHOT,
> >> +				      buffer, npages);
> >> +		ASSERT_EQ(ret, 0);
> >> +		ASSERT_EQ(buffer->cpages, npages);
> >> +
> >> +		m = buffer->mirror;
> >> +		for (i = 0; i < npages; ++i)
> >> +			ASSERT_EQ(m[i], HMM_DMIRROR_PROT_DEV_PRIVATE_LOCAL |
> >> +					HMM_DMIRROR_PROT_WRITE |
> >> +					HMM_DMIRROR_PROT_PMD);
> >
> > madvise(..., MADV_HUGEPAGE) is not sufficient to guarantee that the allocation
> > was indeed converted to THP. Might be worth using the kpageflags interface (but that
> > requires elevated privileges) and then KPF_THP? Otherwise the HMM_DMIRROR_PROT_PMD
> > can be a miss from time to time. One other option is not to assert, but to check
> > and inform?
>
> I'll then use the existing check_huge_anon() to assert that a PMD THP got allocated.

Would MADV_COLLAPSE work here?

>
> >
> >> +
> >> +		pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> >> +		ASSERT_GE(pagemap_fd, 0);
> >> +
> >> +		for (i = 0; i < npages; ++i) {
> >> +			entry = pagemap_get_entry(pagemap_fd,
> >> +					(char *)buffer->ptr + i * self->page_size);
> >> +
> >
> > If this is a THP entry, do we have valid pagemap entries for offset of i * page_size?
>
> Yep we do, see the populate_pagemap label in pagemap_pmd_range_thp.
>
> >
> >> +			ASSERT_NE(entry & PM_SWAP, 0);
> >> +			ASSERT_EQ(entry & PM_PRESENT, 0);
> >
> > Nit: You can use PAGEMAP_PRESENT()
>
> Okay.
>
> >
> >> +		}
> >> +
> >> +		close(pagemap_fd);
> >> +	}
> >> +
> >>  	/* Fault pages back to system memory and check them. */
> >>  	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
> >>  		ASSERT_EQ(ptr[i], i);
> >
> > Balbir Singh
>

Cheers, Lorenzo