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Hi Ryan,

First off, this is excellent work!  Your cover page was very detailed
and made the patch set easier to understand.

Some questions/comments:

Once a kernel is booted with a certain page size, could there be issues
if it is booted later with a different page size?  How about if this is
done frequently?

I think that is the reason why you are only given the option in RHEL
to select the kernel (4K vs. 64K) to use at install time.

Software can easily use a different data format for persistance based
on the base page size. I would suspect DBs might be the usual suspects.

One example is swap space I think, where the base page size used when
formatting the device is used, and it cannot be used with a different
page size unless reformatting it.

So ... one has to be a bit careful ...

Yes, that is what I was thinking.  Once a userspace process does an I/O
and if it is based on PAGE_SIZE things can go south.  I think this is
not an issue with THP, so maybe it's possible with boot-time page selection?

THP is a different beast and has different semantics: the base page size 
doesn't change: the result of getpagesize() is unmodified ("transparent").

One would have to emulate for a given user space process a different 
page size ... and Ryan can likely tell some stories about that.

Not that I consider it reasonable to have dynamic page sizes in the 
kernel and then try emulating a different one for all user space.

-- 
Cheers,

David / dhildenb