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

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Message-ID: <CAPcyv4hS7i1DApKPDB5PkfBNZVbk321FgP94kUDjmuyGXDidZg@mail.gmail.com>
Date:   Mon, 12 Sep 2016 10:29:17 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Linux MM <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-nvdimm@...ts.01.org>,
        linux-api@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <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, Sep 11, 2016 at 8:35 PM, Nicholas Piggin <npiggin@...il.com> wrote:
> 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.

Fair point, I'll beef up the documentation if this moves past an RFC.

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

The new bits are left as zero unless an application explicitly asks
for them, so an existing mincore() user shouldn't break.

> 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?

Certainly one of the new request flags can indicate that the vector is
made up of larger entries.