linux-kernel - Re: [PATCHv2] mm: Fix warning in move_normal

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Message-ID: <20200716131612.b7ottczdgnxwdvrp@box>
Date:   Thu, 16 Jul 2020 16:16:12 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        William Kucharski <william.kucharski@...cle.com>
Subject: Re: [PATCHv2] mm: Fix warning in move_normal_pmd()

On Wed, Jul 15, 2020 at 04:18:44PM -0700, Linus Torvalds wrote:
> On Wed, Jul 15, 2020 at 4:04 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > It *might* be as simple as this incremental thing on top
> 
> No, it needs to be
> 
> +       if (*old_addr + *len < old->vm_end)
> +               return;
> 
> in try_to_align_end(), of course.

Okay, this should work, but I'm not convinced that it gives much: number
of cases covered by the optimization not going to be high.

It can also lead to performance regression: for small mremap() if only one
side of the range got aligned and there's no PMD_SIZE range to move,
kernel will still iterate over PTEs, but it would need to handle more
pte_none()s than without the patch.

-- 
 Kirill A. Shutemov