linux-kernel - Re: [PATCH v2] mm, page_alloc: Fix has_unmovable

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Date:   Thu, 20 Dec 2018 10:12:28 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Wei Yang <richard.weiyang@...il.com>, akpm@...ux-foundation.org,
        vbabka@...e.cz, pavel.tatashin@...rosoft.com,
        rppt@...ux.vnet.ibm.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm, page_alloc: Fix has_unmovable_pages for HugePages

On Thu 20-12-18 00:39:18, Oscar Salvador wrote:
> On Wed, Dec 19, 2018 at 02:25:28PM +0000, Wei Yang wrote:
> > >-			iter = round_up(iter + 1, 1<<compound_order(page)) - 1;
> > >+			skip_pages = (1 << compound_order(head)) - (page - head);
> > >+			iter = round_up(iter + 1, skip_pages) - 1;
> > 
> > The comment of round_up says round up to next specified power of 2.  And
> > second parameter must be a power of 2.
> > 
> > Look skip_pages not satisfy this.

Yes this is true but the resulting numbers should be correct even for
skips that are not power of 2 AFAIC. Or do you have any counter example?

> 
> At least alloc_gigantic_page() looks for 1GB range, aligned to that.
> But I see that in alloc_contig_range(), the boundaries can differ.
> 
> Anyway, unless I am missing something, I think that we could just
> get rid of the round_up() and do something like:
> 
> <--
> skip_pages = (1 << compound_order(head)) - (page - head);
> iter = skip_pages - 1;
> --
> 
> which looks more simple IMHO.

Agreed!

-- 
Michal Hocko
SUSE Labs