Re: [PATCH v2] mm: incorporate zero pages into transparent huge pages

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 18 Feb 2015 19:08:12 -0500 Rik van Riel <riel@redhat.com> wrote:

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> On 02/18/2015 06:31 PM, Andrew Morton wrote:
> > On Wed, 11 Feb 2015 23:03:55 +0200 Ebru Akagunduz
> > <ebru.akagunduz@gmail.com> wrote:
> > 
> >> This patch improves THP collapse rates, by allowing zero pages.
> >> 
> >> Currently THP can collapse 4kB pages into a THP when there are up
> >> to khugepaged_max_ptes_none pte_none ptes in a 2MB range.  This
> >> patch counts pte none and mapped zero pages with the same
> >> variable.
> > 
> > So if I'm understanding this correctly, with the default value of 
> > khugepaged_max_ptes_none (HPAGE_PMD_NR-1), if an application
> > creates a 2MB area which contains 511 mappings of the zero page and
> > one real page, the kernel will proceed to turn that area into a
> > real, physical huge page.  So it consumes 2MB of memory which would
> > not have previously been allocated?
> 
> This is equivalent to an application doing a write fault
> to a 2MB area that was previously untouched, going into
> do_huge_pmd_anonymous_page() and receiving a 2MB page.
> 
> > If so, this might be rather undesirable behaviour in some
> > situations (and ditto the current behaviour for pte_none ptes)?
> > 
> > This can be tuned by adjusting khugepaged_max_ptes_none,
> 
> The example of directly going into do_huge_pmd_anonymous_page()
> is not influenced by the tunable.
> 
> It may indeed be undesirable in some situations, but I am
> not sure how to detect those...

Here's a live one: https://bugzilla.kernel.org/show_bug.cgi?id=93111

Application does MADV_DONTNEED to free up a load of memory and then
khugepaged comes along and pages that memory back in again.  It seems a
bit silly to do this after userspace has deliberately discarded those
pages!

Presumably MADV_NOHUGEPAGE can be used to prevent this, but it's a bit
of a hand-grenade.  I guess the MADV_DONTNEED manpage should be updated
to explain all this?

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