linux-kernel - Re: [PATCH 2/2] mm/mempolicy: Skip extra call to __alloc_pages

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Message-ID: <20250630230147.2280374-1-joshua.hahnjy@gmail.com>
Date: Mon, 30 Jun 2025 16:01:47 -0700
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Kees Bakker <kees@...erbout.nl>,
	Gregory Price <gourry@...rry.net>,
	Alistair Popple <apopple@...dia.com>,
	Byungchul Park <byungchul@...com>,
	David Hildenbrand <david@...hat.com>,
	Matthew Brost <matthew.brost@...el.com>,
	Rakie Kim <rakie.kim@...com>,
	Ying Huang <ying.huang@...ux.alibaba.com>,
	Zi Yan <ziy@...dia.com>,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	kernel-team@...a.com
Subject: Re: [PATCH 2/2] mm/mempolicy: Skip extra call to __alloc_pages_bulk in weighted interleave

On Mon, 30 Jun 2025 15:35:01 -0700 Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 30 Jun 2025 13:21:14 -0700 Joshua Hahn <joshua.hahnjy@...il.com> wrote:
> 
> > > This is a goto into the middle of a for-loop.
> > > What do you think is going to happen at the end of that loop?
> > > 
> > > I think (only tested with a small C program) it will go to the start of
> > > the loop, do the i++, check i<nnodes, and possibly do the loop again.
> > > Variable i is uninitialized at that point. In the loop it hits several
> > > uninitialized variables.
> > 
> > >From what I can see from my code, I think the only the goto statement leads
> > to a second iteration of the for loop is if allocation fails.
> > But otherwise, it should be ok since we always hit
> > 
> > if (total_allocated == nr_pages)
> > 	break;
> > 
> > within the loop. For the branch that takes the goto, we set
> > node_pages = rem_pages, then jump to the label and allocate.
> > So nr_allocated = node_pages, and total_allocated = 0 + nr_allocated
> > so total_allocated = node_pages
> > 
> > total_allocated == node_pages == rem_pages == nr_pages, so we will break. Phew!
> > 
> > To cover the case where allocation fails, I think we should be breaking
> > anyways, so I can definitely add a new check for this.
> 
> I do agree, that goto is a "goto too far".  That we can do a thing
> doesn't mean we should do it!

Haha : -)

> > > Even if this is legal C code, it is pretty obscure.
> > 
> > I agree that it not very clean. I did this to reduce the amount of repeated
> > code there is. Even if this code works, it could definitely be written
> > better to make it more readable and maintainable. As I noted in my second
> > response to Gregory, I'm not planning on pursuing this version anymore,
> > so if I decide to send a second version, I'll keep this in mind.
> 
> Cool, I'll drop this version from mm-unstable.

Sounds good Andrew, thank you always for all of your help!
Joshua

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