Hello, On Wed, Jun 14, 2017 at 04:18:57PM +0200, Martin Schwidefsky wrote: > Could we change pmdp_invalidate to make it return the old pmd entry? That to me seems the simplest fix to avoid losing the dirty bit. I earlier suggested to replace pmdp_invalidate with something like old_pmd = pmdp_establish(pmd_mknotpresent(pmd)) (then tlb flush could then be conditional to the old pmd being present). Making pmdp_invalidate return the old pmd entry would be mostly equivalent to that. The advantage of not changing pmdp_invalidate is that we could skip a xchg which is more costly in __split_huge_pmd_locked and madvise_free_huge_pmd so perhaps there's a point to keep a variant of pmdp_invalidate that doesn't use xchg internally (and in turn can't return the old pmd value atomically). If we don't want new messy names like pmdp_establish we could have a __pmdp_invalidate that returns void, and pmdp_invalidate that returns the old pmd and uses xchg (and it'd also be backwards compatible as far as the callers are concerned). So those places that don't need the old value returned and can skip the xchg, could simply s/pmdp_invalidate/__pmdp_invalidate/ to optimize. One way or another for change_huge_pmd I think we need a xchg like in native_pmdp_get_and_clear but that sets the pmd to pmd_mknotpresent(pmd) instead of zero. And this whole issues originates because both change_huge_pmd(prot_numa = 1) and madvise_free_huge_pmd both run concurrently with the mmap_sem for reading. In the earlier email on this topic, I also mentioned the concern of the _notify mmu notifier invalidate that got dropped silently with the s/pmdp_huge_get_and_clear_notify/pmdp_invalidate/ conversion but I later noticed the mmu notifier invalidate is already covered by the caller. So change_huge_pmd should have called pmdp_huge_get_and_clear in the first place and the _notify prefix in the old code was a mistake as far as I can tell. So we can focus only on the dirty bit retention issue. Thanks, Andrea -- 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>
