Re: [RFC PATCH] mm: thp: Add per-mm_struct flag to control THP

From: David Rientjes
Date: Wed Jan 22 2014 - 16:46:18 EST

On Wed, 22 Jan 2014, Alex Thorlton wrote:

> > I would expect that the alternative solution to a per-mm switch is to
> > reserve the naturally aligned pages for a THP promotion. Have a threshold
> > of pages pages that must be faulted before the full THP's worth of pages
> > is allocated, zero'd and a huge pmd established. That would defer the
> > THP setup costs until it was detected that it was necessary.
> I have some half-finished patches that I was working on a month or so
> ago, to do exactly this (I think you were involved with some of the
> discussion, maybe? I'd have to dig up the e-mails to be sure). After
> cycling through numerous other methods of handling this problem, I still
> like that idea, but I think it's going to require a decent amount of
> effort to get finished.

If you're going to go this route, I think a sane value would be
max_ptes_none that controls when khugepaged would re-collapse a split
hugepage into another hugepage after something does madvise(MADV_DONTNEED)
to a region, for example. Otherwise it could collapse that memory into a
hugepage even though it wasn't faulted as huge.
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