On 12/19/22 5:19 PM, Muhammad Usama Anjum wrote: > Addition of vma_soft_dirty_enabled() has tinkered with the soft-dirty PTE > bit status setting. The internal behavior has changed. The test case was > shared by David > (https://lore.kernel.org/all/bfcae708-db21-04b4-0bbe-712badd03071@xxxxxxxxxx/). > The explanation is as following: > > _Before_ addition of this patch(76aefad628aae), > m = mmap(2 pages) > clear_softdirty() > mremap(m + pag_size) > mprotect(READ) > mprotect(READ | WRITE); > memset(m) > After memset(), > PAGE-1 PAGE-2 > VM_SOFTDIRTY set set > PTE softdirty flag set set > /proc//pagemap view set set > > > _After_ addition of this patch(76aefad628aae) > m = mmap(2 pages) > clear_softdirty() > mremap(m + page_size) > mprotect(READ) > mprotect(READ | WRITE); > memset(m) > After memset(), > PAGE-1 PAGE-2 > VM_SOFTDIRTY set set > PTE softdirty flag *not set* set > /proc//pagemap view set set > > The user's point of view hasn't changed. But internally after this patch, > the soft-dirty tracking in PTEs gets turn off if VM_SOFTDIRTY is set. The > soft-dirty tracking in the PTEs shouldn't be just turned off when mprotect > is used. Why? Because soft-dirty tracking in the PTEs is always enabled > regardless of VM_SOFTDIRTY is set or not. Example: > > m = mem(2 pages) > At this point: > PAGE-1 PAGE-2 > VM_SOFTDIRTY set set > PTE softdirty flag not set not set > /proc//pagemap view set set > memset(m) > At this point: > PAGE-1 PAGE-2 > VM_SOFTDIRTY set set > PTE softdirty flag set set > /proc//pagemap view set set > > This example proves that soft-dirty flag on the PTE is set regardless of > the VM_SOFTDIRTY. Hi Andrew and Cyrill, Peter doesn't agree with me here that this change in behavior should be reverted etc. Please comment. -- BR, Muhammad Usama Anjum
