On 06/11/2018 11:03 AM, Michal Hocko wrote: > On Mon 11-06-18 10:51:44, Jason Baron wrote: >> On 06/11/2018 03:20 AM, Michal Hocko wrote: >>> [CCing linux-api - please make sure to CC this mailing list anytime you >>> are touching user visible apis] >>> >>> On Fri 08-06-18 14:56:52, Jason Baron wrote: >>>> In order to free memory that is marked MLOCK_ONFAULT, the memory region >>>> needs to be first unlocked, before calling MADV_DONTNEED. And if the region >>>> is to be reused as MLOCK_ONFAULT, we require another call to mlock2() with >>>> the MLOCK_ONFAULT flag. >>>> >>>> Let's simplify freeing memory that is set MLOCK_ONFAULT, by allowing >>>> MADV_DONTNEED to work directly for memory that is set MLOCK_ONFAULT. >>> >>> I do not understand the point here. How is MLOCK_ONFAULT any different >>> from the regular mlock here? If you want to free mlocked memory then >>> fine but the behavior should be consistent. MLOCK_ONFAULT is just a way >>> to say that we do not want to pre-populate the mlocked area and do that >>> lazily on the page fault time. madvise should make any difference here. >>> >> >> The difference for me is after the page has been freed, MLOCK_ONFAULT >> will re-populate the range if its accessed again. Whereas with regular >> mlock I don't think it will because its normally done at mlock() or >> mmap() time. > > The vma would still be locked so we would effectively turn it into > ONFAULT IIRC. > Indeed. I just tried allowing MADV_DONTNEED against regular mlock() and in my brief testing it seemed to work as expected against both anonymous and file back pages. I am certainly not against allowing it for regular mlock() as well, if you think that makes it more consistent. >> In any case, the state of a region being locked with >> regular mlock and pages not present does not currently exist, whereas it >> does for MLOCK_ONFAULT, so it seems more natural to do it only for >> MLOCK_ONFAULT. Finally, the use-case we had for this, didn't need >> regular mlock(). > > 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? 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
