On 06/12/2018 03:46 AM, Michal Hocko wrote: > On Mon 11-06-18 12:23:58, Jason Baron wrote: >> On 06/11/2018 11:03 AM, Michal Hocko wrote: >>> So can we start discussing whether we want to allow MADV_DONTNEED on >>> mlocked areas and what downsides it might have? Sure it would turn the >>> strong mlock guarantee to have the whole vma resident but is this >>> acceptable for something that is an explicit request from the owner of >>> the memory? >>> >> >> If its being explicity requested by the owner it makes sense to me. I >> guess there could be a concern about this breaking some userspace that >> relied on MADV_DONTNEED not freeing locked memory? > > Yes, this is always the fear when changing user visible behavior. I can > imagine that a userspace allocator calling MADV_DONTNEED on free could > break. The same would apply to MLOCK_ONFAULT/MCL_ONFAULT though. We > have the new flag much shorter so the probability is smaller but the > problem is very same. So I _think_ we should treat both the same because > semantically they are indistinguishable from the MADV_DONTNEED POV. Both > remove faulted and mlocked pages. Mlock, once applied, should guarantee > no later major fault and MADV_DONTNEED breaks that obviously. > > So the more I think about it the more I am worried about this but I am > more and more convinced that making ONFAULT special is just a wrong way > around this. > Ok, I share the concern that there is a chance that userspace is relying on MADV_DONTNEED not free'ing locked memory. In that case, what if we introduce a MADV_DONTNEED_FORCE, which does everything that MADV_DONTNEED currently does but in addition will also free mlock areas. That way there is no concern about breaking something. Thanks, -Jason
