Re: [PATCH 2/2] mm/compaction: speed up pageblock_pfn_to_page() when zone is contiguous

[Vlastimil Babka]

On 12/23/2015 07:57 AM, Joonsoo Kim wrote:
> > > What are the cases where pageblock_pfn_to_page() is used for a subset of
> > > the pageblock and the result would be problematic for compaction?  I.e.,
> > > do we actually care to use pageblocks that are not contiguous at all?
> >
> > The problematic pageblocks are those that have pages from more than one zone in
> > them, so we just skip them. Supposedly that can only happen by switching once
> > between two zones somewhere in the middle of the pageblock, so it's sufficient
> > to check first and last pfn and compare their zones. So using
> > pageblock_pfn_to_page() on a subset from compaction would be wrong. Holes (==no
> > pages) within pageblock is a different thing checked by pfn_valid_within()
> > (#defined out on archs where such holes cannot happen) when scanning the block.
> >
> > That's why I'm not entirely happy with how the patch conflates both the
> > first/last pfn's zone checks and pfn_valid_within() checks. Yes, a fully
> > contiguous zone does *imply* that pageblock_pfn_to_page() doesn't have to check
> > first/last pfn for a matching zone. But it's not *equality*. And any (now just
> > *potential*) user of pageblock_pfn_to_page() with pfn's different than
> > first/last pfn of a pageblock is likely wrong.
>
> Now, I understand your concern. What makes me mislead is that
> 3 of 4 callers to pageblock_pfn_to_page() in compaction.c could call it with
> non-pageblock boundary pfn.

Oh, I thought you were talking about potential new callers, now that the 
function was exported. So let's see about the existing callers:

isolate_migratepages() - first pfn can be non-boundary when restarting 
from a middle of pageblock, that's true. But it means the pageblock has 
already passed the check in previous call where it was boundary, so it's 
safe. Worst can happen that the restarting pfn will be in a 
intra-pageblock hole so pageblock will be falsely skipped over.

isolate_freepages() - always boundary AFAICS?

isolate_migratepages_range() and isolate_freepages_range() - yeah the 
CMA parts say it doesn't have to be aligned, I don't know about actual users

> Maybe, they should be fixed first.

It would be probably best, even for isolate_migratepages() for 
consistency and less-surprisibility.

> Then, yes. I can
> separate first/last pfn's zone checks and pfn_valid_within() checks.
> If then, would you be entirely happy? :)

Maybe, if the patch also made me a coffee :P

> Thanks.

Thanks!

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