linux-kernel - Re: [RFC PATCH 1/2] mm, mincore2(): retrieve dax and tlb-size attributes of an address range

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Date:   Mon, 12 Sep 2016 13:35:36 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, linux-nvdimm@...ts.01.org,
        linux-api@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm, mincore2(): retrieve dax and tlb-size
 attributes of an address range

On Sun, 11 Sep 2016 10:31:35 -0700
Dan Williams <dan.j.williams@...el.com> wrote:

> As evidenced by this bug report [1], userspace libraries are interested
> in whether a mapping is DAX mapped, i.e. no intervening page cache.
> Rather than using the ambiguous VM_MIXEDMAP flag in smaps, provide an
> explicit "is dax" indication as a new flag in the page vector populated
> by mincore.

Can you cc linux-arch when adding new syscalls (or other such things that
need arch enablement).

I wonder if the changelog for a new syscall should have a bit more grandeur.
Without seeing patch 2, you might not know this was a new syscall just by
reading the subject and changelog.

mincore() defines other bits to be reserved, but I guess it probably breaks
things if you suddenly started using them.

It's a bit sad to introduce a new syscall for this and immediately use up
all bits that can be returned. Would it be a serious problem to return a
larger mask per page?

Thanks,
Nick