linux-kernel - Re: [RFC PATCH 1/1] mm/debug_vm_pgtable: Use a swp_entry

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Message-ID: <e60539ab-e319-4a7c-a7ee-493df3f7f833@arm.com>
Date: Mon, 30 Jun 2025 09:48:04 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: David Hildenbrand <david@...hat.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, LKML <linux-kernel@...r.kernel.org>,
 linux-mm <linux-mm@...ck.org>, linux-s390@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] mm/debug_vm_pgtable: Use a swp_entry_t input
 value for swap tests



On 25/06/25 10:17 PM, David Hildenbrand wrote:
>> [...]
>>>> @@ -1166,6 +1173,7 @@ static void __init init_fixed_pfns(struct pgtable_debug_args *args)
>>>>     static int __init init_args(struct pgtable_debug_args *args)
>>>>   {
>>>> +    unsigned long max_swap_offset;
>>>>       struct page *page = NULL;
>>>>       int ret = 0;
>>>>   @@ -1248,6 +1256,11 @@ static int __init init_args(struct pgtable_debug_args *args)
>>>>         init_fixed_pfns(args);
>>>>   +    /* 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))));
>>> Why not directly use generic_max_swapfile_size() which is doing exact same thing.
>>>
>>> unsigned long generic_max_swapfile_size(void)
>>> {
>>>     return swp_offset(pte_to_swp_entry(
>>>             swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
>>> }
>>
>> Good question. I just moved this code here from pte_swap_exclusive_tests(),
>> see above, and did not think about that. Now I also wonder why
>> generic_max_swapfile_size() wasn't used before.
>>
>> But it is not exactly the same thing, there is an extra "+ 1" there.
>> Maybe that is the reason, but I don't really understand the details /
>> difference, and therefore would not want to change it.
>>
>> David, do you remember why you didn't use generic_max_swapfile_size()
>> in your pte_swap_exclusive_tests()?
> 
> Excellent question. If only I would remember :)
> 
> generic_max_swapfile_size() resides in mm/swapfile.c, which is only around with CONFIG_SWAP.
> 
> It makes sense to have that function only if there are ... actual swapfiles.
> 
> These checks here are independent of CONFIG_SWAP (at least in theory -- for migration entries etc we don't need CONFIG_SWAP), and we simply want to construct a swap PTE with all possible bits set.

After this modification of PMD based swap test - there will be now
two uses for generic_max_swapfile_size(). Rather than refactoring
these into a similar helper in mm/debug_vm_pgtable.c - should the
existing helper just be moved outside of CONFIG_SWAP, thus making
it available in general ?