On Wed 13-06-18 08:32:19, Vlastimil Babka wrote: > 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. But MADV_DONTNEED is an explicit call for data loss. Or do I miss your point? > The madvise manpage is also quite specific about not allowing > MADV_DONTNEED and MADV_FREE for locked pages. Yeah, but that seems to describe the state of the art rather than explain why. > 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? That is what Jason wanted but I argued that the two are the same from MADV_DONTNEED point of view. I do not see how treating them differently would be less confusing or error prone. It's new so we can make it behave differently is certainly not an argument. > >> 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? Well, I wanted to bring this up as well. I guess this would require some more hacks to handle the reclaim path correctly because we do rely on VM_LOCK at many places for the lazy mlock pages culling. -- Michal Hocko SUSE Labs -- 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
