Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages

[Andrea Arcangeli]

Hello,

On Tue, Nov 24, 2020 at 01:32:05PM +0000, Mel Gorman wrote:
> I would hope that is not the case because they are not meant to overlap.
> However, if the beginning of the pageblock was not the start of a zone
> then the pages would be valid but the pfn would still be outside the
> zone boundary. If it was reserved, the struct page is valid but not
> suitable for set_pfnblock_flags_mask. However, it is a concern in
> general because the potential is there that pages are isolated from the
> wrong zone.

I guess we have more than one issue to correct in that function
because the same BUG_ON reproduced again even with the tentative patch
I posted earlier.

So my guess is that the problematic reserved page isn't pointed by the
min_pfn, but it must have been pointed by the "highest" variable
calculated below?

			if (pfn >= highest)
				highest = pageblock_start_pfn(pfn);

When I looked at where "highest" comes from, it lacks
pageblock_pfn_to_page check (which was added around v5.7 to min_pfn).

Is that the real bug, which may be fixed by something like this? (untested)

==