Subject: Re: [PATCH resend v6 02/30] mm: mempolicy: fix interleave index for unaligned VMA start

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 11 May 2026 11:15:24 -0400

On Mon, May 11, 2026 at 10:59:46AM -0400, Gregory Price wrote:
> 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-masked]>
> 
> Reviewed-by: Gregory Price <[email-masked]>
> 
> 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.

Sure Andrew can pick it up if he wants. I found a bunch of unrelated other
"technically broken stuff" that I deferred fixing. Will get to it
once this is done.

> > ---
> >  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