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From: "Huang, Ying" <ying.huang@intel.com>
To: Gregory Price <gregory.price@memverge.com>
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Subject: Re: [PATCH v5 02/11] mm/mempolicy: introduce
 MPOL_WEIGHTED_INTERLEAVE for weighted interleaving
In-Reply-To: <ZZXbN4+2nVbE/lRe@memverge.com> (Gregory Price's message of "Wed,
	3 Jan 2024 17:09:59 -0500")
References: <20231223181101.1954-1-gregory.price@memverge.com>
	<20231223181101.1954-3-gregory.price@memverge.com>
	<8734vof3kq.fsf@yhuang6-desk2.ccr.corp.intel.com>
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	<878r58dt31.fsf@yhuang6-desk2.ccr.corp.intel.com>
	<ZZRybDPSoLme8Ldh@memverge.com>
	<87mstnc6jz.fsf@yhuang6-desk2.ccr.corp.intel.com>
	<ZZXbN4+2nVbE/lRe@memverge.com>
Date: Thu, 04 Jan 2024 13:39:31 +0800
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Gregory Price <gregory.price@memverge.com> writes:

> On Wed, Jan 03, 2024 at 01:46:56PM +0800, Huang, Ying wrote:
>> Gregory Price <gregory.price@memverge.com> writes:
>> > I'm specifically concerned about:
>> > 	weighted_interleave_nid
>> > 	alloc_pages_bulk_array_weighted_interleave
>> >
>> > I'm unsure whether kmalloc/kfree is safe (and non-offensive) in those
>> > contexts. If kmalloc/kfree is safe fine, this problem is trivial.
>> >
>> > If not, there is no good solution to this without pre-allocating a
>> > scratch area per-task.
>> 
>> You need to audit whether it's safe for all callers.  I guess that you
>> need to allocate pages after calling, so you can use the same GFP flags
>> here.
>> 
>
> After picking away i realized that this code is usually going to get
> called during page fault handling - duh.  So kmalloc is almost never
> safe (or can fail), and we it's nasty to try to handle those errors.

Why not just OOM for allocation failure?

> Instead of doing that, I simply chose to implement the scratch space
> in the mempolicy structure
>
> mempolicy->wil.scratch_weights[MAX_NUMNODES].
>
> We eat an extra 1kb of memory in the mempolicy, but it gives us a safe
> scratch space we can use any time the task is allocating memory, and
> prevents the need for any fancy error handling.  That seems like a
> perfectly reasonable tradeoff.

I don't think that this is a good idea.  The weight array is temporary.

--
Best Regards,
Huang, Ying