linux-kernel - [PATCH v3 02/14] mm: Filter zone device pages returned from folio_walk

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Message-ID: <4ecb7b357fc5b435588024770b88bbb695c30090.1750323463.git-series.apopple@nvidia.com>
Date: Thu, 19 Jun 2025 18:57:54 +1000
From: Alistair Popple <apopple@...dia.com>
To: akpm@...ux-foundation.org
Cc: linux-mm@...ck.org,
	Alistair Popple <apopple@...dia.com>,
	gerald.schaefer@...ux.ibm.com,
	dan.j.williams@...el.com,
	jgg@...pe.ca,
	willy@...radead.org,
	david@...hat.com,
	linux-kernel@...r.kernel.org,
	nvdimm@...ts.linux.dev,
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Subject: [PATCH v3 02/14] mm: Filter zone device pages returned from folio_walk_start()

Previously dax pages were skipped by the pagewalk code as pud_special() or
vm_normal_page{_pmd}() would be false for DAX pages. Now that dax pages are
refcounted normally that is no longer the case, so the pagewalk code will
start returning them.

Most callers already explicitly filter for DAX or zone device pages so
don't need updating. However some don't, so add checks to those callers.

Signed-off-by: Alistair Popple <apopple@...dia.com>

---

Changes since v2:

 - Drop filtering in s390 secure fault handling as suggested by David

Changes since v1:

 - Dropped "mm/pagewalk: Skip dax pages in pagewalk" and replaced it
   with this new patch for v2

 - As suggested by David and Jason we can filter the folios in the
   callers instead of doing it in folio_start_walk(). Most callers
   already do this (see below).

I audited all callers of folio_walk_start() and found the following:

mm/ksm.c:

break_ksm() - doesn't need to filter zone_device pages because the can
never be KSM pages.

get_mergeable_page() - already filters out zone_device pages.
scan_get_next_rmap_iterm() - already filters out zone_device_pages.

mm/huge_memory.c:

split_huge_pages_pid() - already checks for DAX with
vma_not_suitable_for_thp_split()

mm/rmap.c:

make_device_exclusive() - only works on anonymous pages, although
there'd be no issue with finding a DAX page even if support was extended
to file-backed pages.

mm/migrate.c:

add_folio_for_migration() - already checks the vma with vma_migratable()
do_pages_stat_array() - explicitly checks for zone_device folios

kernel/event/uprobes.c:

uprobe_write_opcode() - only works on anonymous pages, not sure if
zone_device could ever work so add an explicit check

arch/s390/mm/fault.c:

do_secure_storage_access() - not sure so be conservative and add a check

arch/s390/kernel/uv.c:

make_hva_secure() - not sure so be conservative and add a check
---
 kernel/events/uprobes.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 8a601df..f774367 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -539,7 +539,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
 	}
 
 	ret = 0;
-	if (unlikely(!folio_test_anon(folio))) {
+	if (unlikely(!folio_test_anon(folio) || folio_is_zone_device(folio))) {
 		VM_WARN_ON_ONCE(is_register);
 		folio_put(folio);
 		goto out;
-- 
git-series 0.9.1