On 08/19/2015 11:33 PM, Eric B Munson wrote:
On Wed, 12 Aug 2015, Michal Hocko wrote:
On Sun 09-08-15 01:22:53, Eric B Munson wrote:
I do not like this very much to be honest. We have only few bits
left there and it seems this is not really necessary. I thought that
LOCKONFAULT acts as a modifier to the mlock call to tell whether to
poppulate or not. The only place we have to persist it is
mlockall(MCL_FUTURE) AFAICS. And this can be handled by an additional
field in the mm_struct. This could be handled at __mm_populate level.
So unless I am missing something this would be much more easier
in the end we no new bit in VM flags would be necessary.
This would obviously mean that the LOCKONFAULT couldn't be exported to
the userspace but is this really necessary?
Sorry for the latency here, I was on vacation and am now at plumbers.
I am not sure that growing the mm_struct by another flags field instead
of using available bits in the vm_flags is the right choice.
I was making the same objection on one of the earlier versions and since
you sticked with a new vm flag, I thought it doesn't matter, as we could
change it later if we run out of bits. But now I realize that since you
export this difference to userspace (and below you say that it's by
request), we won't be able to change it later. So it's a more difficult
choice.
After this
patch, we still have 3 free bits on 32 bit architectures (2 after the
userfaultfd set IIRC). The group which asked for this feature here
wants the ability to distinguish between LOCKED and LOCKONFAULT regions
and without the VMA flag there isn't a way to do that.
Do we know that these last two open flags are needed right now or is
this speculation that they will be and that none of the other VMA flags
can be reclaimed?
I think it's the latter, we can expect that flags will be added rather
than removed, as removal is hard or impossible.
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