Re: [Xen-devel] [PATCH] xen/memguard: Drop memguard_init() entirely

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.02.16 at 17:18, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 19/02/16 14:44, Jan Beulich wrote:
>>>>> On 18.02.16 at 19:03, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> It is not obvious what this code is doing.  Most of it dates from 2007/2008,
>>> and there have been substantial changes in Xen's memory handling since then.
>> Deleting code which isn't understood what it is or was once used
>> for is sub-optimal.
>>
>>> It was previously optional, and isn't needed for any of the memguard
>>> infrastructure to function.  The use of MAP_SMALL_PAGES causes needless
>>> shattering of superpages.
>> Perhaps that's what is its purpose? Let's ask Keir, whom you didn't
>> even Cc.
> 
> I don't see this patch being different in nature to your "x86: drop
> failsafe callback invocation from assembly".

That other patch explains why the code is (and never was)
necessary, whereas you just guess.

> As I explain in the second paragraph, these calls are strictly optional,
> as they are omitted for release builds.  They also have no impact on the
> rest of the memguard infrastructure to function, as
> __memguard_change_range() also uses map_pages_to_xen().
> 
> So despite not being sure why it is like it is, I am stating with that
> it is not needed with Xen in its current form.

I actually think the reason is to avoid the memory allocation which
might result the first time a 2M page gets split up, as that memory
allocation might fail (which nowadays gets a proper -ENOMEM
communicated out of map_pages_to_xen(), but that hasn't been
the case in the early days).

Jan


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