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Date: Mon, 19 Feb 2024 13:07:31 +0100
From: Michal Hocko <mhocko@suse.com>
To: Donet Tom <donettom@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org,
	Aneesh Kumar <aneesh.kumar@kernel.org>,
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	Mike Kravetz <mike.kravetz@oracle.com>,
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	Hugh Dickins <hughd@google.com>,
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Subject: Re: [PATCH 3/3] mm/numa_balancing:Allow migrate on protnone
 reference with MPOL_PREFERRED_MANY policy
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On Sat 17-02-24 01:31:35, Donet Tom wrote:
> commit bda420b98505 ("numa balancing: migrate on fault among multiple bound
> nodes") added support for migrate on protnone reference with MPOL_BIND
> memory policy. This allowed numa fault migration when the executing node
> is part of the policy mask for MPOL_BIND. This patch extends migration
> support to MPOL_PREFERRED_MANY policy.
> 
> Currently, we cannot specify MPOL_PREFERRED_MANY with the mempolicy flag
> MPOL_F_NUMA_BALANCING. This causes issues when we want to use
> NUMA_BALANCING_MEMORY_TIERING. To effectively use the slow memory tier,
> the kernel should not allocate pages from the slower memory tier via
> allocation control zonelist fallback. Instead, we should move cold pages
> from the faster memory node via memory demotion. For a page allocation,
> kswapd is only woken up after we try to allocate pages from all nodes in
> the allocation zone list. This implies that, without using memory
> policies, we will end up allocating hot pages in the slower memory tier.
> 
> MPOL_PREFERRED_MANY was added by commit b27abaccf8e8 ("mm/mempolicy: add
> MPOL_PREFERRED_MANY for multiple preferred nodes") to allow better
> allocation control when we have memory tiers in the system. With
> MPOL_PREFERRED_MANY, the user can use a policy node mask consisting only
> of faster memory nodes. When we fail to allocate pages from the faster
> memory node, kswapd would be woken up, allowing demotion of cold pages
> to slower memory nodes.
> 
> With the current kernel, such usage of memory policies implies we can't
> do page promotion from a slower memory tier to a faster memory tier
> using numa fault. This patch fixes this issue.
> 
> For MPOL_PREFERRED_MANY, if the executing node is in the policy node
> mask, we allow numa migration to the executing nodes. If the executing
> node is not in the policy node mask but the folio is already allocated
> based on policy preference (the folio node is in the policy node mask),
> we don't allow numa migration. If both the executing node and folio node
> are outside the policy node mask, we allow numa migration to the
> executing nodes.

The feature makes sense to me. How has this been tested? Do you have any
numbers to present?

> Signed-off-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
> Signed-off-by: Donet Tom <donettom@linux.ibm.com>
> ---
>  mm/mempolicy.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)

I haven't spotted anything obviously wrong in the patch itself but I
admit this is not an area I am actively familiar with so I might be
missing something.
-- 
Michal Hocko
SUSE Labs