On Fri, Dec 19, 2014 at 8:10 AM, Jan Beulich <[email protected]> wrote: > The old scheme can lead to failure in certain cases - the problem is > that after bumping step_size the next (non-final) iteration is only > guaranteed to make available a memory block the size of what step_size > was before. E.g. for a memory block [0,3004600000) we'd have > > iter start end step amount > 1 3004400000 30045fffff 2M 2M > 2 3004000000 30043fffff 64M 4M > 3 3000000000 3003ffffff 2G 64M > 4 2000000000 2fffffffff 64G 64G > > Yet to map 64G with 4k pages (as happens e.g. under PV Xen) we need > slightly over 128M, but the first three iterations made only about 70M > available. > > The condition (new_mapped_ram_size > mapped_ram_size) for bumping > step_size is just not suitable. Instead we want to bump it when we know > we have enough memory available to cover a block of the new step_size. > And rather than making that condition more complicated than needed, > simply adjust step_size by the largest possible factor we know we can > cover at that point - which is shifting it left by one less than the > difference between page table level shifts. (Interestingly the original > STEP_SIZE_SHIFT definition had a comment hinting at that having been > the intention, just that it should have been PUD_SHIFT-PMD_SHIFT-1 > instead of (PUD_SHIFT-PMD_SHIFT)/2, and of course for non-PAE 32-bit we > can't really use these two constants as they're equal there.)
Acked-by: Yinghai Lu <[email protected]> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
