linux-kernel - Re: [PATCH v10 3/4] mm, thp: introduce FOLL_SPLIT

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Date:   Thu, 1 Aug 2019 17:04:33 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>,
        "srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v10 3/4] mm, thp: introduce FOLL_SPLIT_PMD

On 07/31, Song Liu wrote:
>
> > On Jul 31, 2019, at 8:18 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > Now, I don't understand why do we need pmd_trans_unstable() after
> > split_huge_pmd(huge-zero-pmd), but whatever reason we have, why can't we
> > unify both cases?
> >
> > IOW, could you explain why the path below is wrong?
>
> I _think_ the following patch works (haven't fully tested yet). But I am not
> sure whether this is the best. By separating the two cases, we don't duplicate
> much code. And it is clear that the two cases are handled differently.
> Therefore, I would prefer to keep these separate for now.

I disagree. I think this separation makes the code less readable/understandable.
Exactly because it handles two cases differently and it is absolutely not clear
why.

But I can't argue, please forget.

Oleg.