Hi Arnd, On Wed, Aug 22, 2012 at 06:13:10PM +0100, Catalin Marinas wrote: > On Wed, Aug 22, 2012 at 01:27:14PM +0100, Arnd Bergmann wrote: > > On Wednesday 22 August 2012, Catalin Marinas wrote: > > > But what's more important - moving this wrapper to glibc causes issues > > > with the page size. We support both 4KB and 64KB pages on 64-bit systems > > > (the latter without compat support). The kernel is in a better position > > > to do the shift by a compile-time constant. Glibc would need to enquire > > > the actual page size to do the shift before calling sys_mmap_pgoff. If > > > we assume in glibc that the shift is always 12, we need another wrapper > > > in the kernel anyway for 64KB page configuration. So passing the offset > > > in bytes worked best for us. > > > > Right, the kernel interface should really be independent of the page > > size, as sys_mmap2 normally is, and sys_mmap2 is not provided here. > > sys_mmap2 is indeed independent of the page size on most architectures > assuming that the last argument represents the offset in units of 4096. > The cris and ia64 seem to differ (one being 8K, the other variable). > > sys_mmap is also independent of the page size. > > But using sys_mmap2 for a 64-bit architecture, especially when the page > size is not always 4K, does not bring any advantages. We end up doing a > shift by 12 in glibc and another shift by (PAGE_SHIFT - 12) in the > kernel wrapper. Unless I missed your point, I don't see the reason for > using sys_mmap2 on a 64-bit architecture, apart from it being newer (and > compat support should not have any relevance, we have different syscall > tables anyway). I forgot about this at the KS and we haven't got to a clear conclusion. Do we (1) stick with the sys_mmap() for 64-bit systems and avoid offset conversion in both glibc and kernel or (2) use sys_mmap2() with a 12 shift in glibc and (PAGE_SHIFT - 12) in the kernel wrapper? I personally prefer (1) as it doesn't require a kernel wrapper and we avoid the double shifting. A reason for (2) would be if we ever need file offsets greater than 16EB. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
