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.
