Date: Wed, 22 Jan 2014 13:46:04 -0800 (PST)
From: David Rientjes <rientjes@google.com>
To: Alex Thorlton <athorlton@sgi.com>
cc: Mel Gorman <mgorman@suse.de>, Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>, linux-mm@kvack.org,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Rik van Riel <riel@redhat.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Al Viro <viro@zeniv.linux.org.uk>, Kees Cook <keescook@chromium.org>,
        Andrea Arcangeli <aarcange@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] mm: thp: Add per-mm_struct flag to control THP
In-Reply-To: <20140122175328.GO18196@sgi.com>
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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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