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 12:30:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
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 Mon, Jul 05, 2021 at 09:09:22AM +0530, Anshuman Khandual wrote:
> 
> 
> On 7/5/21 8:58 AM, Matthew Wilcox wrote:
> > On Mon, Jul 05, 2021 at 08:57:54AM +0530, Anshuman Khandual wrote:
> >>
> >> 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 ?
> > 
> > No?  It doesn't actually cause any problems, does it?
> 
> No but should an unnecessary element in a struct is dropped only if there
> is a reported problem ?

In this case, yes.  It's not just a struct member; it's a member of a
union in the struct.  You don't save any memory by getting rid of it.
There's no benefit to getting rid of it.