linux-kernel - Re: [PATCH v2 3/5] mm: swap_pte_batch: add an output argument to reture if all swap entries are exclusive

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Message-ID: <a5a55919-46bc-4107-a0fc-14dc404e8c90@redhat.com>
Date: Thu, 11 Apr 2024 17:00:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>,
 akpm@...ux-foundation.org, linux-mm@...ck.org
Cc: baolin.wang@...ux.alibaba.com, chrisl@...nel.org, hanchuanhua@...o.com,
 hannes@...xchg.org, hughd@...gle.com, kasong@...cent.com, surenb@...gle.com,
 v-songbaohua@...o.com, willy@...radead.org, xiang@...nel.org,
 ying.huang@...el.com, yosryahmed@...gle.com, yuzhao@...gle.com,
 ziy@...dia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] mm: swap_pte_batch: add an output argument to
 reture if all swap entries are exclusive

On 11.04.24 16:54, Ryan Roberts wrote:
> On 09/04/2024 09:26, Barry Song wrote:
>> From: Barry Song <v-songbaohua@...o.com>
>>
>> Add a boolean argument named any_shared. If any of the swap entries are
>> non-exclusive, set any_shared to true. The function do_swap_page() can
>> then utilize this information to determine whether the entire large
>> folio can be reused.
>>
>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
>> ---
>>   mm/internal.h | 9 ++++++++-
>>   mm/madvise.c  | 2 +-
>>   mm/memory.c   | 2 +-
>>   3 files changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 9d3250b4a08a..cae39c372bfc 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -238,7 +238,8 @@ static inline pte_t pte_next_swp_offset(pte_t pte)
>>    *
>>    * Return: the number of table entries in the batch.
>>    */
>> -static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
>> +static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte,
>> +				bool *any_shared)
> 
> Please update the docs in the comment above this for the new param; follow
> folio_pte_batch()'s docs as a template.
> 
>>   {
>>   	pte_t expected_pte = pte_next_swp_offset(pte);
>>   	const pte_t *end_ptep = start_ptep + max_nr;
>> @@ -248,12 +249,18 @@ static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
>>   	VM_WARN_ON(!is_swap_pte(pte));
>>   	VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
>>   
>> +	if (any_shared)
>> +		*any_shared |= !pte_swp_exclusive(pte);
> 
> This is different from the approach in folio_pte_batch(). It inits *any_shared
> to false and does NOT include the value of the first pte. I think that's odd,
> personally and I prefer your approach. I'm not sure if there was a good reason
> that David chose the other approach?

Because in my case calling code does

nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags,
		     &any_writable);

..

if (any_writable)
	pte = pte_mkwrite(pte, src_vma);

..

and later checks in another function pte_write().

So if the common pattern is that the original PTE will be used for 
checks, then it doesn't make sense to unnecessary checks+setting for the 
first PTE.

-- 
Cheers,

David / dhildenb