linux-kernel - Re: [PATCH] mm: enforce THP for VM

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Date:   Fri, 28 Sep 2018 15:36:49 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     yang.shi@...ux.alibaba.com
Cc:     Jan Kara <jack@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "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>
Subject: Re: [PATCH] mm: enforce THP for VM_NOHUGEPAGE dax mappings

On Fri, Sep 28, 2018 at 3:34 PM <yang.shi@...ux.alibaba.com> wrote:
>
> commit baabda261424517110ea98c6651f632ebf2561e3 ("mm: always enable thp
> for dax mappings") says madvise hguepage policy makes less sense for
> dax, and force enabling thp for dax mappings in all cases, even though
> THP is set to "never".
>
> However, transparent_hugepage_enabled() may return false if
> VM_NOHUGEPAGE is set even though the mapping is dax.
>
> So, move is_vma_dax() check to the very beginning to enforce THP for dax
> mappings in all cases.
>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
> I didn't find anyone mention the check should be before VM_NOHUGEPAGE in
> the review for Dan's original patch. And, that patch commit log states
> clearly that THP for dax mapping for all cases even though THP is never.
> So, I'm supposed it should behave in this way.

No, if someone explicitly does MADV_NOHUGEPAGE then the kernel should
honor that, even if the mapping is DAX.