On Mon, May 11, 2026 at 05:01:49AM -0400, Michael S. Tsirkin wrote: > The NUMA interleave index formula (addr - vm_start) >> shift > gives wrong results when vm_start is not aligned to the folio > size: the subtraction before the shift allows low bits to > affect the result via borrows. > > Use (addr >> shift) - (vm_start >> shift) instead, which > independently aligns both values before computing the > difference. > > No functional change for current callers: the fix only affects > NUMA interleave and weighted-interleave policies. Current > large-order callers either pre-align the address > (vma_alloc_anon_folio_pmd) or do not use NUMA interleave > (drm_pagemap). All other callers use order 0 where the old > and new formulas are equivalent. However subsequent patches > in this series add large-order callers that pass unaligned > fault addresses, making this fix necessary. > > Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Gregory Price <[email protected]> Should this just be pulled out ahead as a fix (assuming this set takes more time to bake)? I get it's not causing issues today, but the interface is otherwise technicaly broken either way. > --- > mm/mempolicy.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 6832cc68120f..39e556e3d263 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -2049,7 +2049,8 @@ struct mempolicy *get_vma_policy(struct vm_area_struct > *vma, > if (pol->mode == MPOL_INTERLEAVE || > pol->mode == MPOL_WEIGHTED_INTERLEAVE) { > *ilx += vma->vm_pgoff >> order; > - *ilx += (addr - vma->vm_start) >> (PAGE_SHIFT + order); > + *ilx += (addr >> (PAGE_SHIFT + order)) - > + (vma->vm_start >> (PAGE_SHIFT + order)); There's enough (PAGE_SHIFT + ...) spread around the kernel, i wonder if it's worth a define or a function (not in scope, just pondering). ~Gregory
