linux-kernel - Re: [PATCH v9 03/12] mm: deferred_init

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Date:   Tue, 3 Oct 2017 12:01:08 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, kasan-dev@...glegroups.com, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net,
        willy@...radead.org, ard.biesheuvel@...aro.org,
        mark.rutland@....com, will.deacon@....com, catalin.marinas@....com,
        sam@...nborg.org, mgorman@...hsingularity.net,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        bob.picco@...cle.com
Subject: Re: [PATCH v9 03/12] mm: deferred_init_memmap improvements

Hi Michal,

Are you OK, if I replace DEFERRED_FREE() macro with a function like this:

/*
  * Helper for deferred_init_range, free the given range, and reset the
  * counters
  */
static inline unsigned long __def_free(unsigned long *nr_free,
                                        unsigned long *free_base_pfn,
                                        struct page **page)
{
         unsigned long nr = *nr_free;

         deferred_free_range(*free_base_pfn, nr);
         *free_base_pfn = 0;
         *nr_free = 0;
         *page = NULL;

         return nr;
}

Since it is inline, and we operate with non-volatile counters, compiler 
will be smart enough to remove all the unnecessary de-references. As a 
plus, we won't be adding any new branches, and the code is still going 
to stay compact.

Pasha

On 10/03/2017 11:15 AM, Pasha Tatashin wrote:
> Hi Michal,
> 
>>
>> Please be explicit that this is possible only because we discard
>> memblock data later after 3010f876500f ("mm: discard memblock data
>> later"). Also be more explicit how the new code works.
> 
> OK
> 
>>
>> I like how the resulting code is more compact and smaller.
> 
> That was the goal :)
> 
>> for_each_free_mem_range also looks more appropriate but I really detest
>> the DEFERRED_FREE thingy. Maybe we can handle all that in a single goto
>> section. I know this is not an art but manipulating variables from
>> macros is more error prone and much more ugly IMHO.
> 
> Sure, I can re-arrange to have a goto place. Function won't be as small, 
> and if compiler is not smart enough we might end up with having more 
> branches than what my current code has.
> 
>>
>> please do not use macros. Btw. this deserves its own fix. I suspect that
>> no CONFIG_HOLES_IN_ZONE arch enables DEFERRED_STRUCT_PAGE_INIT but
>> purely from the review point of view it should be its own patch.
> 
> Sure, I will submit this patch separately from the rest of the project. 
> In my opinion DEFERRED_STRUCT_PAGE_INIT is the way of the future, so we 
> should make sure it is working with as many configs as possible.
> 
> Thank you,
> Pasha