On 30.06.25 07:18, Muhammad Usama Anjum wrote:
On 6/17/25 7:35 PM, David Hildenbrand wrote:
is_zero_pfn() does not work for the huge zero folio. Fix it by using
is_huge_zero_pmd().
Found by code inspection.
Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
Probably we should Cc stable, thoughts?
We should also extend the pagemap_ioctl selftest to cover this case, but I
don't have time for that right now. @Muhammad ?
Currently, we don't have any test case covering zero pfn. I'm trying to write
a few test cases. But I'm not able to get ZERO PFN. I've tried to allocate a
read only memory and then read it. Is there a trick to how to create ZERO PFN
memory from userspace?
You need a MAP_ANON | MAP_PRIVATE mapping and have to make sure that the
compiler does not optimize out the read.
E.g.,
char *mem = mmap(...);
char tmp = *mem;
asm volatile("" : "+r" (tmp))
or
char *mem = mmap(...);
*(volatile char *)mem;
To get the shared huge zero folio, you need a suitably aligned VMA. See
run_with_huge_zeropage() in in tools/testing/selftests/mm/cow.c as one
example.
--
Cheers,
David / dhildenb
