On Mon, Nov 13, 2017 at 08:14:54PM +0100, Thomas Gleixner wrote: > On Mon, 13 Nov 2017, Thomas Gleixner wrote: > > On Mon, 13 Nov 2017, Kirill A. Shutemov wrote: > > > > > On Mon, Nov 13, 2017 at 04:43:26PM +0100, Thomas Gleixner wrote: > > > > On Tue, 7 Nov 2017, Kirill A. Shutemov wrote: > > > > > > > > > In case of 5-level paging, we don't put any mapping above 47-bit, unless > > > > > userspace explicitly asked for it. > > > > > > > > > > Userspace can ask for allocation from full address space by specifying > > > > > hint address above 47-bit. > > > > > > > > > > Nicholas noticed that current implementation violates this interface: > > > > > we can get vma partly in high addresses if we ask for a mapping at very > > > > > end of 47-bit address space. > > > > > > > > > > Let's make sure that, when consider hint address for non-MAP_FIXED > > > > > mapping, start and end of resulting vma are on the same side of 47-bit > > > > > border. > > > > > > > > What happens for mappings with MAP_FIXED which cross the border? > > > > > > It will succeed with 5-level paging. > > > > And why is this allowed? > > > > > It should be safe as with 4-level paging such request would fail and it's > > > reasonable to expect that userspace is not relying on the failure to > > > function properly. > > > > Huch? > > > > The first rule when looking at user space is that is broken or > > hostile. Reasonable and user space are mutually exclusive. > > Aside of that in case of get_unmapped_area: > > If va_unmapped_area() fails, then the address and the len which caused the > overlap check to trigger are handed in to arch_get_unmapped_area(), which > again can create an invalid mapping if I'm not missing something. > > If mappings which overlap the boundary are invalid then we have to make > sure at all ends that they wont happen. They are not invalid. The patch tries to address following theoretical issue: We have an application that tries, for some reason, to allocate memory with mmap(addr), without MAP_FIXED, where addr is near the borderline of 47-bit address space and addr+len is above the border. On 4-level paging machine this request would succeed, but the address will always be within 47-bit VA -- cannot allocate by hint address, ignore it. If the application cannot handle high address this might be an issue on 5-level paging machine as such call would succeed *and* allocate memory by the specified hint address. In this case part of the mapping would be above the border line and may lead to misbehaviour. I hope this makes any sense :) -- Kirill A. Shutemov -- 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>
