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

[Andrea Arcangeli]

On Wed, Nov 25, 2020 at 10:30:53AM +0000, Mel Gorman wrote:
> On Tue, Nov 24, 2020 at 03:56:22PM -0500, Andrea Arcangeli wrote:
> > 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)
> > 
> 
> It's plausible as it is a potential source of leaking but as you note
> in another mail, it's surprising to me that valid struct pages, even if
> within memory holes and reserved would have broken node/zone information
> in the page flags.

I think the patch to add pageblock_pfn_to_page is still needed to cope
with !pfn_valid or a pageblock in between zones, but pfn_valid or
pageblock in between zones is not what happens here.

So the patch adding pageblock_pfn_to_page would have had the undesired
side effect of hiding the bug so it's best to deal with the other bug
first.