On 03/26/2015 08:39 PM, Davide Libenzi wrote: > On Thu, 26 Mar 2015, David Rientjes wrote: > >> Yes, this munmap() behavior of lengths <= hugepage_size - PAGE_SIZE for a >> hugetlb vma is long standing and there may be applications that break as a >> result of changing the behavior: a database that reserves all allocated >> hugetlb memory with mmap() so that it always has exclusive access to those >> hugepages, whether they are faulted or not, and maintains its own hugepage >> pool (which is common), may test the return value of munmap() and depend >> on it returning -EINVAL to determine if it is freeing memory that was >> either dynamically allocated or mapped from the hugetlb reserved pool. > > You went a long way to create such a case. > But, in your case, that application will erroneously considering hugepage > mmaped memory, as dynamically allocated, since it will always get EINVAL, > unless it passes an aligned size. Aligned size, which a fix like the one > posted in the patch will still leave as success. > OTOH, an application, which might be more common than the one you posted, > which calls munmap() to release a pointer which it validly got from a > previous mmap(), will leak huge pages as all the issued munmaps will fail. > > >> If we were to go back in time and decide this when the munmap() behavior >> for hugetlb vmas was originally introduced, that would be valid. The >> problem is that it could lead to userspace breakage and that's a >> non-starter. >> >> What we can do is improve the documentation and man-page to clearly >> specify the long-standing behavior so that nobody encounters unexpected >> results in the future. > > This way you will leave the mmap API with broken semantics. > In any case, I am done arguing. > I will leave to Andrew to sort it out, and to Michael Kerrisk to update > the mmap man pages with the new funny behaviour. + CC's You know that people don't always magically CC themselves, or read all of lkml/linux-mm? :) > > > - Davide > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- 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
