linux-kernel - Re: [PATCH V2] mm/page_alloc: Add alloc_contig

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Message-ID: <20191016124149.GB317@dhcp22.suse.cz>
Date:   Wed, 16 Oct 2019 14:41:49 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm/page_alloc: Add alloc_contig_pages()

On Wed 16-10-19 14:29:05, David Hildenbrand wrote:
> On 16.10.19 13:51, Michal Hocko wrote:
> > On Wed 16-10-19 16:43:57, Anshuman Khandual wrote:
> > > 
> > > 
> > > On 10/16/2019 04:39 PM, David Hildenbrand wrote:
> > [...]
> > > > Just to make sure, you ignored my comment regarding alignment
> > > > although I explicitly mentioned it a second time? Thanks.
> > > 
> > > I had asked Michal explicitly what to be included for the respin. Anyways
> > > seems like the previous thread is active again. I am happy to incorporate
> > > anything new getting agreed on there.
> > 
> > Your patch is using the same alignment as the original code would do. If
> > an explicit alignement is needed then this can be added on top, right?
> > 
> 
> Again, the "issue" I see here is that we could now pass in numbers that are
> not a power of two. For gigantic pages it was clear that we always have a
> number of two. The alignment does not make any sense otherwise.
> 
> What I'm asking for is
> 
> a) Document "The resulting PFN is aligned to nr_pages" and "nr_pages should
> be a power of two".

OK, this makes sense.

> b) Eventually adding something like
> 
> if (WARN_ON_ONCE(!is_power_of_2(nr_pages)))
> 	return NULL;

I am not sure this is really needed.

-- 
Michal Hocko
SUSE Labs