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Date:   Wed, 2 Feb 2022 13:18:54 +0100
From:   Oscar Salvador <osalvador@suse.de>
To:     Zi Yan <ziy@nvidia.com>
Cc:     David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        linuxppc-dev@lists.ozlabs.org,
        virtualization@lists.linux-foundation.org,
        iommu@lists.linux-foundation.org, Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Eric Ren <renzhengeek@gmail.com>
Subject: Re: [PATCH v4 3/7] mm: page_isolation: check specified range for
 unmovable pages
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References: <20220119190623.1029355-1-zi.yan@sent.com>
 <20220119190623.1029355-4-zi.yan@sent.com>
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On Wed, Jan 19, 2022 at 02:06:19PM -0500, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> Enable set_migratetype_isolate() to check specified sub-range for
> unmovable pages during isolation. Page isolation is done
> at max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages) granularity, but not all
> pages within that granularity are intended to be isolated. For example,
> alloc_contig_range(), which uses page isolation, allows ranges without
> alignment. This commit makes unmovable page check only look for
> interesting pages, so that page isolation can succeed for any
> non-overlapping ranges.

Another thing that came to my mind.
Prior to this patch, has_unmovable_pages() was checking on pageblock
granularity, starting at pfn#0 of the pageblock.
With this patch, you no longer check on pageblock granularity, which
means you might isolate a pageblock, but some pages that sneaked in
might actually be unmovable.

E.g:

Let's say you have a pageblock that spans (pfn#512,pfn#1024),
and you pass alloc_contig_range() (pfn#514,pfn#1024).
has_unmovable_pages() will start checking the pageblock at pfn#514,
and so it will mis pfn#512 and pfn#513. Isn't that a problem, if those
pfn turn out to be actually unmovable?


-- 
Oscar Salvador
SUSE Labs