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

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Message-Id: <20250630153501.64160f386faa541c93344e48@linux-foundation.org>
Date: Mon, 30 Jun 2025 15:35:01 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Joshua Hahn <joshua.hahnjy@...il.com>
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 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!

> > 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.