On Sun, Aug 27, 2023 at 10:42:44AM +0100, Lorenzo Stoakes wrote: > On Tue, Aug 22, 2023 at 01:54:58AM +0000, Joel Fernandes (Google) wrote: > > This patch adds support for verifying that we correctly handle the > > situation where something is already mapped before the destination of the remap. > > > > Any realignment of destination address and PMD-copy will destroy that > > existing mapping. In such cases, we need to avoid doing the optimization. > > > > To test this, we map an area called the preamble before the remap > > region. Then we verify after the mremap operation that this region did not get > > corrupted. > > > > Putting some prints in the kernel, I verified that we optimize > > correctly in different situations: > > > > Optimize when there is alignment and no previous mapping (this is tested > > by previous patch). > > <prints> > > can_align_down(old_vma->vm_start=2900000, old_addr=2900000, mask=-2097152): 0 > > can_align_down(new_vma->vm_start=2f00000, new_addr=2f00000, mask=-2097152): 0 > > === Starting move_page_tables === > > Doing PUD move for 2800000 -> 2e00000 of extent=200000 <-- Optimization > > Doing PUD move for 2a00000 -> 3000000 of extent=200000 > > Doing PUD move for 2c00000 -> 3200000 of extent=200000 > > </prints> > > > > Don't optimize when there is alignment but there is previous mapping > > (this is tested by this patch). > > Notice that can_align_down() returns 1 for the destination mapping > > as we detected there is something there. > > <prints> > > can_align_down(old_vma->vm_start=2900000, old_addr=2900000, mask=-2097152): 0 > > can_align_down(new_vma->vm_start=5700000, new_addr=5700000, mask=-2097152): 1 > > === Starting move_page_tables === > > Doing move_ptes for 2900000 -> 5700000 of extent=100000 <-- Unoptimized > > Doing PUD move for 2a00000 -> 5800000 of extent=200000 > > Doing PUD move for 2c00000 -> 5a00000 of extent=200000 > > </prints> > > > > Have you additionally tested this by changing the code to be intentionally > broken then running the test and observing it fail? Yes I did! Because while developing the patch, it was broken many times and the test failed during those times. ;-) > Looks good to me, > Reviewed-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> Thanks! - Joel
