On Thu, May 23, 2024 at 9:21 AM Peter Xu <peterx@xxxxxxxxxx> wrote: > > Macro RANDOM_ORVALUE was used to make sure the pgtable entry will be > populated with !none data in clear tests. > > The RANDOM_ORVALUE tried to cover mostly all the bits in a pgtable entry, > even if there's no discussion on whether all the bits will be vaild. Both > S390 and PPC64 have their own masks to avoid touching some bits. Now it's > the turn for x86_64. > > The issue is there's a recent report from Mikhail Gavrilov showing that > this can cause a warning with the newly added pte set check in commit > 8430557fc5 on writable v.s. userfaultfd-wp bit, even though the check > itself was valid, the random pte is not. We can choose to mask more bits > out. > > However the need to have such random bits setup is questionable, as now > it's already guaranteed to be true on below: > > - For pte level, the pgtable entry will be installed with value from > pfn_pte(), where pfn points to a valid page. Hence the pte will be > !none already if populated with pfn_pte(). > > - For upper-than-pte level, the pgtable entry should contain a directory > entry always, which is also !none. > > All the cases look like good enough to test a pxx_clear() helper. Instead > of extending the bitmask, drop the "set random bits" trick completely. Add > some warning guards to make sure the entries will be !none before clear(). > > Cc: David Hildenbrand <david@xxxxxxxxxx> > Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> > Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx> > Cc: Gavin Shan <gshan@xxxxxxxxxx> > Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx> > Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx> > Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx> > Link: https://lore.kernel.org/r/CABXGCsMB9A8-X+Np_Q+fWLURYL_0t3Y-MdoNabDM-Lzk58-DGA@xxxxxxxxxxxxxx > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > --- > mm/debug_vm_pgtable.c | 31 +++++-------------------------- > 1 file changed, 5 insertions(+), 26 deletions(-) Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
