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

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Message-ID: <20191017073413.GC24485@dhcp22.suse.cz>
Date:   Thu, 17 Oct 2019 09:34:13 +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 Thu 17-10-19 09:21:24, David Hildenbrand wrote:
> On 17.10.19 09:11, Michal Hocko wrote:
> > On Thu 17-10-19 10:44:41, Anshuman Khandual wrote:
> > [...]
> > > Does this add-on documentation look okay ? Should we also mention about the
> > > possible reduction in chances of success during pfn block search for the
> > > non-power-of-two cases as the implicit alignment will probably turn out to
> > > be bigger than nr_pages itself ?
> > > 
> > >   * Requested nr_pages may or may not be power of two. The search for suitable
> > >   * memory range in a zone happens in nr_pages aligned pfn blocks. But in case
> > >   * when nr_pages is not power of two, an implicitly aligned pfn block search
> > >   * will happen which in turn will impact allocated memory block's alignment.
> > >   * In these cases, the size (i.e nr_pages) and the alignment of the allocated
> > >   * memory will be different. This problem does not exist when nr_pages is power
> > >   * of two where the size and the alignment of the allocated memory will always
> > >   * be nr_pages.
> > 
> > I dunno, it sounds more complicated than really necessary IMHO. Callers
> > shouldn't really be bothered by memory blocks and other really deep
> > implementation details.. Wouldn't be the below sufficient?
> > 
> > The allocated memory is always aligned to a page boundary. If nr_pages
> > is a power of two then the alignement is guaranteed to be to the given
> 
> s/alignement/alignment/
> 
> and "the PFN is guaranteed to be aligned to nr_pages" (the address is
> aligned to nr_pages*PAGE_SIZE)

thx for the correction.

> > nr_pages (e.g. 1GB request would be aligned to 1GB).
> > 
> 
> I'd probably add "This function will miss allocation opportunities if
> nr_pages is not a power of two (and the implicit alignment is bogus)."

This is again an implementation detail and quite a confusing one to
whoever not familiar with the MM internals. And to be fair even a proper
alignment doesn't give you any stronger guarantee as long as the
allocation operates on non movable zones anyway.
-- 
Michal Hocko
SUSE Labs