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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/16/21 1:34 PM, Vlastimil Babka wrote:
> 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

Clarification. This is true only for !CONFIG_SPARSEMEM, which is unlikely in
practice to produce the configurations that trigger this issue. So we can remove
the VM_BUG_ON_PAGE()

> 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.
> 
> 
> 
>