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: <ccbcbbb5-24eb-4503-ac7b-3171d39536e7@arm.com>
Date: Thu, 11 Apr 2024 16:36:16 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.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 11/04/2024 16:00, David Hildenbrand wrote:
> 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.

Yep understood. And I think adopting your semantics for any_shared actually
simplifies the code in patch 4 too; I've just commented that.