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 [off-list ref] wrote:

On Sun, 11 Sep 2016 10:31:35 -0700
Dan Williams [off-list ref] wrote:

quoted

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.

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