linux-kernel - Re: [RFC PATCH 04/39] mm: mempolicy: Refactor out policy_node

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Message-ID: <ZuHJUN4GDw7vU3Vv@PC2K9PVX.TheFacebook.com>
Date: Wed, 11 Sep 2024 12:46:08 -0400
From: Gregory Price <gourry@...rry.net>
To: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH 04/39] mm: mempolicy: Refactor out
 policy_node_nodemask()

On Tue, Sep 10, 2024 at 11:43:35PM +0000, Ackerley Tng wrote:
> This was refactored out of huge_node().
> 
> huge_node()'s interpretation of vma for order assumes the
> hugetlb-specific storage of the hstate information in the
> inode. policy_node_nodemask() does not assume that, and can be used
> more generically.
> 
> This refactoring also enforces that nid default to the current node
> id, which was not previously enforced.
> 
> alloc_pages_mpol_noprof() is the last remaining direct user of
> policy_nodemask(). All its callers begin with nid being the current
> node id as well. More refactoring is required for to simplify that.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>

Reviewed-by: Gregory Price <gourry@...rry.net>

> +/**
> + * policy_node_nodemask(@mpol, @gfp_flags, @ilx, @nodemask)
> + * @mpol: the memory policy to interpret. Reference must be taken.
> + * @gfp_flags: for this request
> + * @ilx: interleave index, for use only when MPOL_INTERLEAVE or
> + *       MPOL_WEIGHTED_INTERLEAVE
> + * @nodemask: (output) pointer to nodemask pointer for 'bind' and 'prefer-many'
> + *            policy
> + *
> + * Returns a nid suitable for a page allocation and a pointer. If the effective
> + * policy is 'bind' or 'prefer-many', returns a pointer to the mempolicy's
> + * @nodemask for filtering the zonelist.

Technically it's possible for nid to contain MAX_NUMNODES upon return
if weighted interleave is used and the nodemask is somehow invalid
(contains no nodes, including the local node). I would expect this to
be indicative of a larger problem (i.e. should functionally never happen).

Now that I'm looking at it, it's possible the weighted interleave path
should default to returning numa_node_id() if node == MAX_NUMNODES, which
would not require any changes to this patch.

> + */
> +int policy_node_nodemask(struct mempolicy *mpol, gfp_t gfp_flags,
> +			 pgoff_t ilx, nodemask_t **nodemask)
> +{
> +	int nid = numa_node_id();
> +	*nodemask = policy_nodemask(gfp_flags, mpol, ilx, &nid);
> +	return nid;
> +}
> +
>  #ifdef CONFIG_HUGETLBFS
>  /*
>   * huge_node(@vma, @addr, @gfp_flags, @mpol)