linux-kernel - Re: [PATCH] mm: Reduce memory bloat with THP

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Date:   Fri, 15 Dec 2017 13:00:24 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Nitin Gupta <nitin.m.gupta@...cle.com>
Cc:     linux-mm@...ck.org, steven.sistare@...cle.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Nadav Amit <namit@...are.com>,
        Minchan Kim <minchan@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        SeongJae Park <sj38.park@...il.com>, Shaohua Li <shli@...com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Rik van Riel <riel@...hat.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Jan Kara <jack@...e.cz>, Dave Jiang <dave.jiang@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Matthew Wilcox <willy@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>, Tobin C Harding <me@...in.cc>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Reduce memory bloat with THP

On Thu, Dec 14, 2017 at 05:28:52PM -0800, Nitin Gupta wrote:
> Currently, if the THP enabled policy is "always", or the mode
> is "madvise" and a region is marked as MADV_HUGEPAGE, a hugepage
> is allocated on a page fault if the pud or pmd is empty.  This
> yields the best VA translation performance, but increases memory
> consumption if some small page ranges within the huge page are
> never accessed.
> 
> An alternate behavior for such page faults is to install a
> hugepage only when a region is actually found to be (almost)
> fully mapped and active.  This is a compromise between
> translation performance and memory consumption.  Currently there
> is no way for an application to choose this compromise for the
> page fault conditions above.
> 
> With this change, when an application issues MADV_DONTNEED on a
> memory region, the region is marked as "space-efficient". For
> such regions, a hugepage is not immediately allocated on first
> write.  Instead, it is left to the khugepaged thread to do
> delayed hugepage promotion depending on whether the region is
> actually mapped and active. When application issues
> MADV_HUGEPAGE, the region is marked again as non-space-efficient
> wherein hugepage is allocated on first touch.

I think this would be NAK. At least in this form.

What performance testing have you done? Any numbers?

Making whole vma "space_efficient" just because somebody freed one page
from it is just wrong. And there's no way back after this.

> 
> Orabug: 26910556

Wat?

-- 
 Kirill A. Shutemov