Re: [Qemu-devel] [PATCH v3 24/25] tcg: Allocate a guard page after code_gen_buffer

[Peter Maydell]

On 23 September 2015 at 13:00, Richard Henderson <address@hidden> wrote:
> On 09/23/2015 12:39 PM, Peter Maydell wrote:
>> I think we're now doing the MADV_HUGEPAGE over "buffer size
>> minus a page" rather than "buffer size". Does that mean
>> we've gone from doing the madvise on a whole number of
>> hugepages to doing it on something that's not a whole number
>> of hugepages, and if so does the kernel decide not to use
>> hugepages here?
>
> On the whole I don't think it matters.  The static buffer isn't page aligned 
> to
> begin with, much less hugepage aligned, so the fact that we're allocating a
> round number like 32mb here doesn't really mean much.  The beginning and/or 
> end
> pages of the buffer definitely aren't going to be hugepage.
>
> Worse, the same is true for the mmap path, since I've never seen the kernel
> select a hugepage aligned address.  You'd think that adding MAP_HUGEPAGE would
> be akin to MADV_HUGEPAGE, with the additional hint that the address should be
> appropriately aligned for the hugepage, but no.  It implies forced use of
> something from the hugepage pool and that requires extra suid capabilities.
>
> I've wondered about over-allocating on the mmap path, so that we can choose 
> the
> hugepage aligned subregion.  But as far as I can tell, my kernel doesn't
> allocate hugepages at all, no matter what we do.  So it seems a little silly 
> to
> go so far out of the way to get an aligned buffer.

This raises the converse question of "why are we bothering with
MADV_HUGEPAGE at all?" :-)

-- PMM