Subject: Re: [PATCH v4 3/7] mm: page_isolation: check specified range for unmovable pages

From: Oscar Salvador <[email-masked]>
Date: Wed, 2 Feb 2022 13:18:54 +0100

On Wed, Jan 19, 2022 at 02:06:19PM -0500, Zi Yan wrote:
> From: Zi Yan <[email-masked]>
> 
> 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