linux-kernel - Re: [PATCH V2] mm/thp: Make ALLOC_SPLIT_PTLOCKS dependent on USE_SPLIT_PTE

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Date:   Mon, 5 Jul 2021 08:57:54 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm/thp: Make ALLOC_SPLIT_PTLOCKS dependent on
 USE_SPLIT_PTE_PTLOCKS


On 7/1/21 6:27 PM, Matthew Wilcox wrote:
> On Thu, Jul 01, 2021 at 10:51:27AM +0530, Anshuman Khandual wrote:
>>
>>
>> On 5/20/21 4:47 PM, Matthew Wilcox wrote:
>>> On Wed, May 19, 2021 at 01:03:06PM +0530, Anshuman Khandual wrote:
>>>> Split ptlocks need not be defined and allocated unless they are being used.
>>>> ALLOC_SPLIT_PTLOCKS is inherently dependent on USE_SPLIT_PTE_PTLOCKS. This
>>>> just makes it explicit and clear. While here drop the spinlock_t element
>>>> from the struct page when USE_SPLIT_PTE_PTLOCKS is not enabled.
>>>
>>> I didn't spot this email yesterday.  I'm not a fan.  Isn't struct page
>>> already complicated enough without adding another ifdef to it?  Surely
>>> there's a better way than this.
>>
>> This discussion thread just got dropped off the radar, sorry about it.
>> None of the spinlock_t elements are required unless split ptlocks are
>> in use. I understand your concern regarding yet another #ifdef in the
>> struct page definition. But this change is simple and minimal. Do you
>> have any other particular alternative in mind which I could explore ?
> 
> Do nothing?  I don't understand what problem you're trying to solve.

Currently there is an element (spinlock_t ptl) in the struct page for page
table lock. Although a struct page based spinlock is not even required in
case USE_SPLIT_PTE_PTLOCKS evaluates to be false. Is not that something to
be fixed here i.e drop the splinlock_t element if not required ?

The problem is USE_SPLIT_PTE_PTLOCKS and ALLOC_SPLIT_PTLOCKS get evaluated
independently, although they are inherently dependent. ALLOC_SPLIT_PTLOCKS
could just be set to 0, when USE_SPLIT_PTE_PTLOCKS evaluates to be 0. This
patch makes that dependency explicit and also fixes the above situation.