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

[Gregory Price (Meta)]

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.

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