On Tue, 26 Aug 2014, Cyrill Gorcunov wrote: > On Tue, Aug 26, 2014 at 06:43:55PM +0300, Kirill A. Shutemov wrote: > > On Tue, Aug 26, 2014 at 07:18:13PM +0400, Cyrill Gorcunov wrote: > > > > Basically, it's safe if only soft-dirty is allowed to modify vm_flags > > > > without down_write(). But why is soft-dirty so special? > > > > > > because how we use this bit, i mean in normal workload this bit won't > > > be used intensively i think so it's not widespread in kernel code > > > > Weak argument to me. Yes. However rarely it's modified, we don't want any chance of it corrupting another flag. VM_SOFTDIRTY is special in the sense that it's maintained in a very different way from the other VM_flags. If we had a little alignment padding space somewhere in struct vm_area_struct, I think I'd jump at Kirill's suggestion to move it out of vm_flags and into a new field: that would remove some other special casing, like the vma merge issue. But I don't think we have such padding space, and we'd prefer not to bloat struct vm_area_struct for it; so maybe it should stay for now. Besides, with Peter's patch, we're also talking about the locking on modifications to vm_page_prot, aren't we? > > > > What about walk through vmas twice: first with down_write() to modify > > vm_flags and vm_page_prot, then downgrade_write() and do > > walk_page_range() on every vma? > > I still it's undeeded, Yes, so long as nothing else is doing the same. No bug yet, that we can see, but a bug in waiting. > but for sure using write-lock/downgrade won't hurt, > so no argues from my side. Yes, Kirill's two-stage suggestion seems the best: down_write quickly scan vmas clearing VM_SOFT_DIRTY and updating vm_page_prot downgrade_write (or up_write, down_read?) slowly walk page tables write protecting and clearing soft-dirty on ptes up_read But please don't mistake me for someone who has a good grasp of soft-dirty: I don't. Hugh -- 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>