linux-kernel - Re: [PATCH v1 1/4] mm/compaction: enable compacting >0 order folios.

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Date:   Mon, 13 Nov 2023 18:30:36 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Zi Yan <ziy@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Huang, Ying" <ying.huang@...el.com>,
        Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Kemeng Shi <shikemeng@...weicloud.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rohan Puri <rohan.puri15@...il.com>,
        Mcgrof Chamberlain <mcgrof@...nel.org>,
        Adam Manzanares <a.manzanares@...sung.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Subject: Re: [PATCH v1 1/4] mm/compaction: enable compacting >0 order folios.

On Mon, Nov 13, 2023 at 12:01:54PM -0500, Zi Yan wrote:
> +	/* this makes migrate_pages() split the source page and retry */
> +	if (folio_order(src) > 0)
> +		return NULL;

Nit: folio_test_large() is more efficient than folio_order() > 0.
The former simply tests the bit, while the second tests the bit, then
loads folio->_order to check it's >0.  We know it will be, but there's
no way to tell gcc that if the bit is set, this value is definitely not 0.