linux-kernel - Re: [PATCH] mm/isolation: Remove redundant pfn_valid_within() in __first_valid

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Message-ID: <20190321100316.GN8696@dhcp22.suse.cz>
Date:   Thu, 21 Mar 2019 11:03:16 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, mike.kravetz@...cle.com,
        zi.yan@...rutgers.edu, akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/isolation: Remove redundant pfn_valid_within() in
 __first_valid_page()

On Thu 21-03-19 10:42:40, Oscar Salvador wrote:
> On Thu, Mar 21, 2019 at 09:43:15AM +0530, Anshuman Khandual wrote:
> > pfn_valid_within() calls pfn_valid() when CONFIG_HOLES_IN_ZONE making it
> > redundant for both definitions (w/wo CONFIG_MEMORY_HOTPLUG) of the helper
> > pfn_to_online_page() which either calls pfn_valid() or pfn_valid_within().
> > pfn_valid_within() being 1 when !CONFIG_HOLES_IN_ZONE is irrelevant either
> > way. This does not change functionality.
> > 
> > Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> 
> About the "Fixes:" tag issue, I agree with Michal that the code is not
> really broken, but perhaps "suboptimal" depending on how much can affect
> performance on those systems where pfn_valid_within() is more complicated than
> simple returning true.
> 
> I see that on arm64, that calls memblock_is_map_memory()->memblock_search(),
> to trigger a search for the region containing the address, so I guess it
> is an expensive operation.
> 
> Depending on how much time we can shave, it might be worth to have the tag
> Fixes, but the removal of the code is fine anyway, so:

Yeah, seeing a noticesable slowdown (actual numbers) would warrant a
backport to 5.0.

-- 
Michal Hocko
SUSE Labs