linux-kernel - Re: [PATCH v15 3/7] mm: Add function __putback_isolated

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Date:   Tue, 17 Dec 2019 11:58:13 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        kvm@...r.kernel.org, mst@...hat.com, linux-kernel@...r.kernel.org,
        willy@...radead.org, mhocko@...nel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        vbabka@...e.cz
Cc:     yang.zhang.wz@...il.com, nitesh@...hat.com, konrad.wilk@...cle.com,
        pagupta@...hat.com, riel@...riel.com, lcapitulino@...hat.com,
        dave.hansen@...el.com, wei.w.wang@...el.com, aarcange@...hat.com,
        pbonzini@...hat.com, dan.j.williams@...el.com, osalvador@...e.de
Subject: Re: [PATCH v15 3/7] mm: Add function __putback_isolated_page

On 16.12.19 17:22, Alexander Duyck wrote:
> On Mon, 2019-12-16 at 12:36 +0100, David Hildenbrand wrote:
>> [...]
>>> +/**
>>> + * __putback_isolated_page - Return a now-isolated page back where we got it
>>> + * @page: Page that was isolated
>>> + * @order: Order of the isolated page
>>> + *
>>> + * This function is meant to return a page pulled from the free lists via
>>> + * __isolate_free_page back to the free lists they were pulled from.
>>> + */
>>> +void __putback_isolated_page(struct page *page, unsigned int order)
>>> +{
>>> +	struct zone *zone = page_zone(page);
>>> +	unsigned long pfn;
>>> +	unsigned int mt;
>>> +
>>> +	/* zone lock should be held when this function is called */
>>> +	lockdep_assert_held(&zone->lock);
>>> +
>>> +	pfn = page_to_pfn(page);
>>> +	mt = get_pfnblock_migratetype(page, pfn);
>>
>> IMHO get_pageblock_migratetype() would be nicer - I guess the compiler
>> will optimize out the double page_to_pfn().
> 
> The thing is I need the page_to_pfn call already in order to pass the
> value to __free_one_page. With that being the case why not juse use
> get_pfnblock_migratetype?

I was reading
	set_pageblock_migratetype(page, migratetype);
And wondered why we don't use straight forward
	get_pageblock_migratetype()
but instead something that looks like a micro-optimization

> 
> Also there are some scenarios where __page_to_pfn is not that simple a
> call with us having to get the node ID so we can find the pgdat structure
> to perform the calculation. I'm not sure the compiler would be ble to
> figure out that the result is the same for both calls, so it is better to
> make it explicit.

Only in case of CONFIG_SPARSEMEM we have to go via the section - but I
doubt this is really worth optimizing here.

But yeah, I'm fine with this change, only "IMHO
get_pageblock_migratetype() would be nicer" :)

> 
>>> +
>>> +	/* Return isolated page to tail of freelist. */
>>> +	__free_one_page(page, pfn, zone, order, mt);
>>> +}
>>> +
>>>  /*
>>>   * Update NUMA hit/miss statistics
>>>   *
>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>> index 04ee1663cdbe..d93d2be0070f 100644
>>> --- a/mm/page_isolation.c
>>> +++ b/mm/page_isolation.c
>>> @@ -134,13 +134,11 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>>  		__mod_zone_freepage_state(zone, nr_pages, migratetype);
>>>  	}
>>>  	set_pageblock_migratetype(page, migratetype);
>>> +	if (isolated_page)
>>> +		__putback_isolated_page(page, order);
>>>  	zone->nr_isolate_pageblock--;
>>>  out:
>>>  	spin_unlock_irqrestore(&zone->lock, flags);
>>> -	if (isolated_page) {
>>> -		post_alloc_hook(page, order, __GFP_MOVABLE);
>>> -		__free_pages(page, order);
>>> -	}
>>
>> So If I get it right:
>>
>> post_alloc_hook() does quite some stuff like
>> - arch_alloc_page(page, order);
>> - kernel_map_pages(page, 1 << order, 1)
>> - kasan_alloc_pages()
>> - kernel_poison_pages(1)
>> - set_page_owner()
>>
>> Which free_pages_prepare() will undo, like
>> - reset_page_owner()
>> - kernel_poison_pages(0)
>> - arch_free_page()
>> - kernel_map_pages()
>> - kasan_free_nondeferred_pages()
>>
>> Both would be skipped now - which sounds like the right thing to do IMHO
>> (and smells like quite a performance improvement). I haven't verified if
>> actually everything we skip in free_pages_prepare() is safe (I think it
>> is, it seems to be mostly relevant for actually used/allocated pages).
> 
> That was kind of my thought on this. Basically the logic I was following
> was that the code path will call move_freepages_block that bypasses all of
> the above mentioned calls if the pages it is moving will not be merged. If
> it is safe in that case my assumption is that it should be safe to just
> call __putback_isolated_page in such a case as it also bypasses the block
> above, but it supports merging the page with other pages that are already
> on the freelist.

Makes sense to me

Acked-by: David Hildenbrand <david@...hat.com>


-- 
Thanks,

David / dhildenb