Re: [PATCH mm-unstable v1 01/26] mm/debug_vm_pgtable: more pte_swp_exclusive() sanity checks

David Hildenbrand
2023-01-14T16:10:09Z

On 13.01.23 18:10, David Hildenbrand wrote:
> We want to implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all architectures.
> Let's extend our sanity checks, especially testing that our PTE bit
> does not affect:
> * is_swap_pte() -> pte_present() and pte_none()
> * the swap entry + type
> * pte_swp_soft_dirty()
>
> Especially, the pfn_pte() is dodgy when the swap PTE layout differs
> heavily from ordinary PTEs. Let's properly construct a swap PTE from
> swap type+offset.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---

The following fixup for !CONFIG_SWAP on top, which makes it compile for me and
passes when booting on x86_64 with CONFIG_DEBUG_VM_PGTABLE:

...
[ 0.347112] Loaded X.509 cert 'Build time autogenerated kernel key: ee6afc0578f6475656fec8a4f9d02832'
[ 0.350112] debug_vm_pgtable: [debug_vm_pgtable ]: Validating architecture page table helpers
[ 0.351217] page_owner is disabled
...

From 6a6162e8af62a4b3f7b9d823fdfae86de3f34a9d Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Sat, 14 Jan 2023 16:47:12 +0100
Subject: [PATCH] fixup: mm/debug_vm_pgtable: more pte_swp_exclusive() sanity
checks

generic_max_swapfile_size() is only available with CONFIG_SWAP -- which
makes sense, because without SWAP there are no swap files. Let's
simply probe manually which bits we can obtain after storing them in a
PTE, and properly call it "max swap offset", which is more generic for
a swap entry.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/debug_vm_pgtable.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 3da0cc380c35..af59cc7bd307 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c
@@ -810,15 +810,17 @@ static void __init pmd_swap_soft_dirty_tests(struct pgtable_debug_args *args) {

static void __init pte_swap_exclusive_tests(struct pgtable_debug_args *args)
{
- unsigned long max_swapfile_size = generic_max_swapfile_size();
+ unsigned long max_swap_offset;
swp_entry_t entry, entry2;
pte_t pte;

pr_debug("Validating PTE swap exclusive\n");

+ /* See generic_max_swapfile_size(): probe the maximum offset */
+ max_swap_offset = swp_offset(pte_to_swp_entry(swp_entry_to_pte(swp_entry(0, ~0UL))));
+
/* Create a swp entry with all possible bits set */
- entry = swp_entry((1 << MAX_SWAPFILES_SHIFT) - 1,
- max_swapfile_size - 1);
+ entry = swp_entry((1 << MAX_SWAPFILES_SHIFT) - 1, max_swap_offset);

pte = swp_entry_to_pte(entry);
WARN_ON(pte_swp_exclusive(pte));
--
2.39.0

--
Thanks,

David / dhildenb