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

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Message-ID: <db8ab8d0-20f5-4922-a1e2-6f7409747664@redhat.com>
Date: Wed, 25 Jun 2025 18:47:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Anshuman Khandual <anshuman.khandual@....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

> [...]
>>> @@ -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.

-- 
Cheers,

David / dhildenb