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Subject: Re: [PATCH v5] mm: Proactive compaction
To:     =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= <holger@applied-asynchrony.com>,
        Nitin Gupta <nigupta@nvidia.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        David Rientjes <rientjes@google.com>,
        Nitin Gupta <ngupta@nitingupta.dev>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Linux API <linux-api@vger.kernel.org>
References: <20200518181446.25759-1-nigupta@nvidia.com>
 <27b39956-2a21-8eef-8ebb-cb3a93a41a36@applied-asynchrony.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <ab3e4c38-a7f9-01fc-f25f-f13f8e30985f@suse.cz>
Date:   Thu, 28 May 2020 11:50:40 +0200
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On 5/28/20 11:15 AM, Holger Hoffstätte wrote:
> 
> On 5/18/20 8:14 PM, Nitin Gupta wrote:
> [patch v5 :)]
> 
> I've been successfully using this in my tree and it works great, but a friend
> who also uses my tree just found a bug (actually an improvement ;) due to the
> change from HUGETLB_PAGE_ORDER to HPAGE_PMD_ORDER in v5.
> 
> When building with CONFIG_TRANSPARENT_HUGEPAGE=n (for some reason it was off)
> HPAGE_PMD_SHIFT expands to BUILD_BUG() and compilation fails like this:

Oops, I forgot about this. Still I believe HPAGE_PMD_ORDER is the best choice as
long as THP's are enabled. I guess fallback to HUGETLB_PAGE_ORDER would be
possible if THPS are not enabled, but AFAICS some architectures don't define
that. Such architectures perhaps won't benefit from proactive compaction anyway?

> ...
> ./include/linux/huge_mm.h:284:28: note: in expansion of macro ‘BUILD_BUG’
>    284 | #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
>        |                            ^~~~~~~~~
> ./include/linux/huge_mm.h:78:26: note: in expansion of macro ‘HPAGE_PMD_SHIFT’
>     78 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
>        |                          ^~~~~~~~~~~~~~~
> mm/compaction.c:1874:28: note: in expansion of macro ‘HPAGE_PMD_ORDER’
>   1874 |    extfrag_for_order(zone, HPAGE_PMD_ORDER);
>        |                            ^~~~~~~~~~~~~~~
> ...
> 
> It would be great if the whole thing would compile without THP; the only
> occurrence is in fragmentation_score_zone(). Unfortunately I'm not familiar
> enough with how to properly check for THP and properly calculate whatever
> you're doing there, otherwise I would ifdef this away myself. ;)
> 
> Thanks for an otherwise great patch!
> 
> cheers,
> Holger
>