linux-kernel - Re: [PATCH v3] mm/page_owner: fix for deferred struct page init

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Date:   Thu, 3 Jan 2019 12:38:59 -0500
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, Pavel.Tatashin@...rosoft.com,
        mingo@...nel.org, hpa@...or.com, mgorman@...hsingularity.net,
        iamjoonsoo.kim@....com, yang.shi@...aro.org, tglx@...utronix.de,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/page_owner: fix for deferred struct page init

On 1/3/19 11:59 AM, Michal Hocko wrote:
>> As mentioned above, "If deselected DEFERRED_STRUCT_PAGE_INIT, it is still better
>> to call page_ext_init() earlier, so page owner could catch more early page
>> allocation call sites."
> 
> Do you have any numbers to show how many allocation are we losing that
> way? In other words, do we care enough to create an ugly code?

Well, I don't have any numbers, but I read that Joonsoo did not really like to
defer page_ext_init() unconditionally.

"because deferring page_ext_init() would make page owner which uses page_ext
miss some early page allocation callsites. Although it already miss some early
page allocation callsites, we don't need to miss more."

https://lore.kernel.org/lkml/20160524053714.GB32186@js1304-P5Q-DELUXE/

>>>> diff --git a/mm/page_ext.c b/mm/page_ext.c
>>>> index ae44f7adbe07..d76fd51e312a 100644
>>>> --- a/mm/page_ext.c
>>>> +++ b/mm/page_ext.c
>>>> @@ -399,9 +399,8 @@ void __init page_ext_init(void)
>>>>  			 * -------------pfn-------------->
>>>>  			 * N0 | N1 | N2 | N0 | N1 | N2|....
>>>>  			 *
>>>> -			 * Take into account DEFERRED_STRUCT_PAGE_INIT.
>>>>  			 */
>>>> -			if (early_pfn_to_nid(pfn) != nid)
>>>> +			if (pfn_to_nid(pfn) != nid)
>>>>  				continue;
>>>>  			if (init_section_page_ext(pfn, nid))
>>>>  				goto oom;
>>>
>>> Also this doesn't seem to be related, right?
>>
>> No, it is related. Because of this patch, page_ext_init() is called after all
>> the memory has already been initialized,
>> so no longer necessary to call early_pfn_to_nid().
> 
> Yes, but it looks like a follow up cleanup/optimization to me.

That early_pfn_to_nid() was introduced in fe53ca54270 (mm: use early_pfn_to_nid
in page_ext_init) which also messed up the order of page_ext_init() in
start_kernel(), so this patch basically revert that commit.