On 08.02.22 00:47, Mike Kravetz wrote: > On 2/4/22 00:35, David Hildenbrand wrote: >>> I thought this was simple. :) >> >> It really bugs me that it's under-specified what's supposed to happen >> when the length is not aligned. >> >> BUT: in the posix world, "calling posix_madvise() shall not affect the >> semantics of access to memory in the specified range". So we don't care >> too much about if we align up/down, because it wouldn't affect the >> semantics. Especially for MADV_DONTNEED/MADV_REMOVE as implemented by >> Linux this is certainly different and the alignment handling matters. >> >> So I guess especially for MADV_DONTNEED/MADV_REMOVE we need a clear >> specification what's supposed to happen if the length falls into the >> middle of a huge page. We should document alignment handling for >> madvise() in general I assume. >> >> IMHO we should have bailed out right from the start whenever something >> is not properly aligned, but that ship has sailed. So I agree, maybe we >> can make at least hugetlb MADV_DONTNEED obey the same (weird) rules as >> ordinary pages. >> >> So b) would mean, requiring start to be hugepage aligned and aligning-up >> the end. Still feels wrong but at least matches existing semantics. >> >> Hugetlb MADV_REMOVE semantics are unfortunate and we should document the >> exception. > > Thank you for all your comments David! > > So, my plan was to make MADV_DONTNEED behave as described above: > - EINVAL if start address not huge page size aligned > - Align end/length up to huge page size. > > The code I had for this was very specific to MADV_DONTNEED. I then thought, > why not do the same for MADV_REMOVE as well? Or even more general, add this > check and alignment to the vma parsing code in madvise. > > It was then that I realized there are several madvise behaviors that take > non-huge page size aligned addresses for hugetlb mappings today. Making > huge page size alignment a requirement for all madvise behaviors could break > existing code. So, it seems like it could only be added to MADV_DONTNEED as > this functionality does not exist today. We then end up with MADV_DONTNEED > as the only behavior requiring huge page size alignment for hugetlb mappings. > Sigh!!! :/ > > I am now rethinking the decision to proceed with b) as described above. > > With the exception of MADV_REMOVE (which we may be able to change for > hugetlb), madvise operations operate on huge page size pages for hugetlb > mappings. If start address is in the middle of a hugetlb page, we essentially > align down to the beginning of the hugetlb page. If length lands in the > middle of a hugetlb page, we essentially round up. Which MADV calls would be affected? The "bad" thing about MADV_DONTNEED and MADV_REMOVE are that they destroy data, which is why they heavily diverge from the original posix madvise odea. > > When adding MADV_REMOVE perhaps we should go with this align down start and > align up end strategy that is used everywhere else? I really wish we could > go back and change things, but as you know it is too late for that. I assume whatever we do, we should document it at least cleanly in the man page. Unfortunately, hugetlb is a gift that keeps on giving. Making it at least somehow consistent, even if it's "hugtlb being consistent in its own mess", that would be preferable I guess. -- Thanks, David / dhildenb
