linux-kernel - Re: [PATCH v5 02/11] mm/mempolicy: introduce MPOL_WEIGHTED

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Date: Thu, 04 Jan 2024 13:39:31 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Gregory Price <gregory.price@...verge.com>
Cc: Gregory Price <gourry.memverge@...il.com>,  <linux-mm@...ck.org>,
  <linux-doc@...r.kernel.org>,  <linux-fsdevel@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>,  <linux-api@...r.kernel.org>,
  <x86@...nel.org>,  <akpm@...ux-foundation.org>,  <arnd@...db.de>,
  <tglx@...utronix.de>,  <luto@...nel.org>,  <mingo@...hat.com>,
  <bp@...en8.de>,  <dave.hansen@...ux.intel.com>,  <hpa@...or.com>,
  <mhocko@...nel.org>,  <tj@...nel.org>,  <corbet@....net>,
  <rakie.kim@...com>,  <hyeongtak.ji@...com>,  <honggyu.kim@...com>,
  <vtavarespetr@...ron.com>,  <peterz@...radead.org>,
  <jgroves@...ron.com>,  <ravis.opensrc@...ron.com>,
  <sthanneeru@...ron.com>,  <emirakhur@...ron.com>,  <Hasan.Maruf@....com>,
  <seungjun.ha@...sung.com>,  Srinivasulu Thanneeru
 <sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v5 02/11] mm/mempolicy: introduce
 MPOL_WEIGHTED_INTERLEAVE for weighted interleaving

Gregory Price <gregory.price@...verge.com> writes:

> On Wed, Jan 03, 2024 at 01:46:56PM +0800, Huang, Ying wrote:
>> Gregory Price <gregory.price@...verge.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