linux-kernel - Re: [PATCH 0/4] further cleanup of vma

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Date:   Mon, 20 Mar 2023 17:09:41 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        maple-tree@...ts.infradead.org
Subject: Re: [PATCH 0/4] further cleanup of vma_merge()

On Mon, Mar 20, 2023 at 12:47:07PM -0400, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lstoakes@...il.com> [230318 07:15]:
> > Following on from Vlastimil Babka's patch series "cleanup vma_merge() and
> > improve mergeability tests" which was in turn based on Liam's prior
> > cleanups, this patch series introduces changes discussed in review of
> > Vlastimil's series and goes further in attempting to make the logic as
> > clear as possible.
> >
> > Nearly all of this should have absolutely no functional impact, however it
> > does add a singular VM_WARN_ON() case.
>
> Thanks for looking at this function and adding more clarity.  I'm happy
> to have comments within the code, especially tricky areas.  But I find
> that adding almost 50 lines to this function makes it rather hard to
> follow.
>
> Can we remove the more obvious comments and possibly reduce the nesting
> of others so there are less lines?
>
> For example in patch 2:
>         /*
>          * If there is a previous VMA specified, find the next, otherwise find
>          * the first.
>          */
>         vma = find_vma(mm, prev ? prev->vm_end : 0);
>
> Is rather verbose for something that can be seen in the code itself.
>
> I think we are risking over-documenting what is going on here which is
> making the code harder to read; the function is pushing 200 lines now.
>
> >
> > Lorenzo Stoakes (4):
> >   mm/mmap/vma_merge: further improve prev/next VMA naming
> >   mm/mmap/vma_merge: set next to NULL if not applicable
> >   mm/mmap/vma_merge: extend invariants, avoid invalid res, vma
> >   mm/mmap/vma_merge: be explicit about the non-mergeable case
> >
> >  mm/mmap.c | 165 +++++++++++++++++++++++++++++++++++-------------------
> >  1 file changed, 107 insertions(+), 58 deletions(-)
> >
> > --
> > 2.39.2
>

Sure, I did try not to overdo things (once you start simplifying you can go
too far), but it seems like I _did_ go too far on the commenting (perhaps
pushing too far the other way).

I will simplify, remove things implied by the code and strip down + respin.