quoted

I think we should not swallo such return values in
isolate_or_dissolve_huge_page() and instead properly report esp. -ENOMEM
properly up this callchain now. Otherwise we'll end up retrying / reporting
-EBUSY, which is misleading.

I am not sure I follow you here.
So, atm, alloc_and_dissolve_huge_page can either generate -ENOMEM or
-EBUSY wrt. error codes.
-ENOMEM when we cannot allocate a page, and -EBUSY when we raced with
someone.
You mean to only report ENOMEM down the chain?

Yes, fatal errors.

quoted

 From isolate_migratepages_range()/isolate_migratepages_block() we'll keep
reporting "pfn > 0".

a) In isolate_migratepages_range() we'll keep iterating over pageblocks
although we should just fail with -ENOMEM right away.

b) In __alloc_contig_migrate_range() we'll keep retrying up to 5 times
although we should just fail with -ENOMEM. We end up returning "-EBUSY"
after retrying.

c) In alloc_contig_range() we'll continue trying to isolate although we
should just return -ENOMEM.

Yes, "fatal" errors get masked, and hence we treat everything as "things
are busy, let us try again", and this is rather unforunate.

quoted

I think we have should start returning proper errors from
isolate_migratepages_range()/isolate_migratepages_block() on critical issues
(-EINTR, -ENOMEM) instead of going via "!pfn vs. pfn" and retrying on "pfn".

So we should then fail with -ENOMEM during isolate_migratepages_range()
cleanly, just as we would do when we get -ENOMEM during migrate_pages().

I guess we could rework the interface and make isolate_migratepages_range and
isolate_migratepages_block to report the right thing.
I yet have to check that this does not mess up a lot with the compaction
interface.

But overall I agree with your point here, and I am willing to to tackle
this.

The question is whether we want to do this as part of this series, or
after this series gets picked up.
IMHO, we could do it after, as a follow-up, unless you feel strong about
it.

What do you think?

I think this is now the second fatal error we can have (-EINTR, 
-ENOMEM), thus the current interface (return "NULL" on fatal errros) no 
longer works properly.

No strong opinion about fixing this up on top - could be it's cleaner to 
just do it right away.

-- 
Thanks,

David / dhildenb