On 06/12/2018 04:11 PM, Jason Baron wrote: > > > 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. I think more concerning than guaranteeing no later major fault is possible data loss, e.g. replacing data with zero-filled pages. The madvise manpage is also quite specific about not allowing MADV_DONTNEED and MADV_FREE for locked pages. So I don't think we should risk changing that for all mlocked pages. Maybe we can risk MCL_ONFAULT, since it's relatively new and has few users? >> 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. A new niche case flag? Sad :( BTW I didn't get why we should allow this for MADV_DONTNEED but not MADV_FREE. Can you expand on that? > Thanks, > > -Jason > -- > To unsubscribe from this list: send the line "unsubscribe linux-api" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html >
