Re: [PATCH] mm/mempolicy: apply bind policies against the scoped allocation zone

[Joshua Hahn]

On Sat, Oct 17, 2026 at 10:33 PM Gregory Price (Meta) <gourry@xxxxxxxxxx> wrote:
>
> QEMU fails to start a guest with a VFIO device when guest RAM comes
> from a memory backend bound to a movable-only node:
>
>   vfio_container_dma_map(0x55b1b1317800, 0x0, 0x10000000,
>                          0x7f39a2200000)
>     = -12 (Cannot allocate memory)
>
> VFIO pins guest RAM when the device is realized, and the pin faults
> the pages in. io_uring fixed buffers fail the same way. Faulting the
> range in before pinning lets the same pin succeed, since resident
> pages are migrated off the node instead.
>
> A long-term pin faults under memalloc_pin_save(). PF_MEMALLOC_PIN makes
> current_gfp_context() clear __GFP_MOVABLE, but policy_nodemask() tests
> apply_policy_zone() against the caller's gfp.
>
> The bind keeps its movable-only nodemask, and the allocation finds no
> usable zone:
>
>   page allocation failure:
>       order:0,
>       mode:0x140dc2(GFP_HIGHUSER|__GFP_ZERO|__GFP_COMP),
>       nodemask=1,cpuset=/,mems_allowed=0-1
>
>   Call Trace:
>    warn_alloc+0x136/0x150
>    __alloc_frozen_pages_noprof+0xed8/0x1720
>    alloc_pages_mpol+0xaa/0x170
>    vma_alloc_folio_noprof+0x65/0xd0
>    __handle_mm_fault+0x137b/0x1ac0
>    handle_mm_fault+0xa0/0x270
>    __get_user_pages+0x250/0xfd0
>    __gup_longterm_locked+0x116/0x870
>    gup_fast_fallback+0xcfc/0xdf0
>    pin_user_pages_fast+0x47/0x60
>    io_pin_pages+0x9a/0x110
>
> Base page faults fail with -ENOMEM.
> hugetlb faults fail with -EFAULT.
>
> Check the policy zone against current_gfp_context(gfp), so a pin-scoped
> fault falls back off the movable-only nodemask, like any other
> non-movable allocation under the same bind.

Hi Gregory,

LGTM, thank you for making it so easy to review!
I wonder if other places are missing current_gfp_context as well.

Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>

> Tested in qemu with a dax/kmem movable-only node 1. io_uring and VFIO
> pins of unfaulted base pages and 2MB hugetlb pages now succeed and land
> on node 0 (correct). QEMU now starts with the same backend binding.
>
> Fixes: 8e3560d963d2 ("mm: honor PF_MEMALLOC_PIN for all movable pages")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> ---
>  mm/mempolicy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 70298fded1b4a..0884c629670dd 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2302,7 +2302,7 @@ static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *pol,
>                 break;
>         case MPOL_BIND:
>                 /* Restrict to nodemask (but not on lower zones) */
> -               if (apply_policy_zone(pol, gfp_zone(gfp)) &&
> +               if (apply_policy_zone(pol, gfp_zone(current_gfp_context(gfp))) &&
>                     cpuset_nodemask_valid_mems_allowed(&pol->nodes))
>                         nodemask = &pol->nodes;
>                 if (pol->home_node != NUMA_NO_NODE)
> --
> 2.53.0-Meta
>