On Wed, Jun 09, 2021 at 11:38:11PM -0700, Hugh Dickins wrote: > page_vma_mapped_walk() cleanup: use pmd_read_atomic() with barrier() > instead of READ_ONCE() for pmde: some architectures (e.g. i386 with PAE) > have a multi-word pmd entry, for which READ_ONCE() is not good enough. > > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > --- > mm/page_vma_mapped.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index 7c0504641fb8..973c3c4e72cc 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -182,13 +182,16 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > pud = pud_offset(p4d, pvmw->address); > if (!pud_present(*pud)) > return false; > + > pvmw->pmd = pmd_offset(pud, pvmw->address); > /* > * Make sure the pmd value isn't cached in a register by the > * compiler and used as a stale value after we've observed a > * subsequent update. > */ > - pmde = READ_ONCE(*pvmw->pmd); > + pmde = pmd_read_atomic(pvmw->pmd); > + barrier(); > + Hm. It makes me wounder if barrier() has to be part of pmd_read_atomic(). mm/hmm.c uses the same pattern as you are and I tend to think that the rest of pmd_read_atomic() users may be broken. Am I wrong? -- Kirill A. Shutemov
