linux-kernel - Re: [PATCH mm-unstable v1 01/26] mm/debug_vm_pgtable: more pte_swp

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Date:   Sat, 14 Jan 2023 17:09:35 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org, openrisc@...ts.librecores.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-um@...ts.infradead.org, linux-xtensa@...ux-xtensa.org
Subject: Re: [PATCH mm-unstable v1 01/26] mm/debug_vm_pgtable: more
 pte_swp_exclusive() sanity checks

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@...hat.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@...hat.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@...el.com>
Signed-off-by: David Hildenbrand <david@...hat.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