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

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Date: Wed, 3 Jan 2024 17:09:59 -0500
From: Gregory Price <gregory.price@...verge.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
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	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

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.

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.

> >
> > Weights are collected individually onto the stack because we have to sum
> > them up before we actually apply the weights.
> >
> > A stale weight is not offensive.  RCU is not needed and doesn't help.
> 
> When you copy weights from iw_table[] to stack, it's possible for
> compiler to cache its contents in register, or merge, split the memory
> operations.  At the same time, iw_table[] may be changed simultaneously
> via sysfs interface.  So, we need a mechanism to guarantee that we read
> the latest contents consistently.
> 

Fair enough, I went ahead and added a similar interaction.

~Gregoryg