Re: [PATCH v5 1/1] mm: refactor initialization of struct page for holes in memory layout

[Vlastimil Babka]

On 2/16/21 12:01 PM, Mike Rapoport wrote:
>> 
>> I do understand that. And I am not objecting to the patch. I have to
>> confess I haven't digested it yet. Any changes to early memory
>> intialization have turned out to be subtle and corner cases only pop up
>> later. This is almost impossible to review just by reading the code.
>> That's why I am asking whether we want to address the specific VM_BUG_ON
>> first with something much less tricky and actually reviewable. And
>> that's why I am asking whether dropping the bug_on itself is safe to do
>> and use as a hot fix which should be easier to backport.
> 
> I can't say I'm familiar enough with migration and compaction code to say
> if it's ok to remove that bug_on. It does point to inconsistency in the
> memmap, but probably it's not important.

On closer look, removing the VM_BUG_ON_PAGE() in set_pfnblock_flags_mask() is
not safe. If we violate the zone_spans_pfn condition, it means we will write
outside of the pageblock bitmap for the zone, and corrupt something. Actually
similar thing can happen in __get_pfnblock_flags_mask() where there's no
VM_BUG_ON, but there we can't corrupt memory. But we could theoretically fault
to do accessing some unmapped range?

So the checks would have to become unconditional !DEBUG_VM and return instead of
causing a BUG. Or we could go back one level and add some checks to
fast_isolate_around() to detect a page from zone that doesn't match cc->zone.
The question is if there is another code that will break if a page_zone()
suddenly changes e.g. in the middle of the pageblock - __pageblock_pfn_to_page()
assumes that if first and last page is from the same zone, so are all pages in
between, and the rest relies on that. But maybe if Andrea's
fast_isolate_around() issue is fixed, that's enough for stable backport.