On Fri, Aug 05, 2022 at 01:48:35PM -0700, Mike Kravetz wrote: > On 08/05/22 20:57, David Hildenbrand wrote: > > On 05.08.22 20:33, Mike Kravetz wrote: > > > On 08/05/22 20:25, David Hildenbrand wrote: > > >> On 05.08.22 20:23, Mike Kravetz wrote: > > >>> On 08/05/22 14:14, Peter Xu wrote: > > >>>> On Fri, Aug 05, 2022 at 01:03:28PM +0200, David Hildenbrand wrote: > > >>>>> diff --git a/mm/mmap.c b/mm/mmap.c > > >>>>> index 61e6135c54ef..462a6b0344ac 100644 > > >>>>> --- a/mm/mmap.c > > >>>>> +++ b/mm/mmap.c > > >>>>> @@ -1683,6 +1683,13 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot) > > >>>>> if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED))) > > >>>>> return 0; > > >>>>> > > >>>>> + /* > > >>>>> + * Hugetlb does not require/support writenotify; especially, it does not > > >>>>> + * support softdirty tracking. > > >>>>> + */ > > >>>>> + if (is_vm_hugetlb_page(vma)) > > >>>>> + return 0; > > >>>> > > >>>> I'm kind of confused here.. you seems to be fixing up soft-dirty for > > >>>> hugetlb but here it's explicitly forbidden. > > >>>> > > >>>> Could you explain a bit more on why this patch is needed if (assume > > >>>> there'll be a working) patch 2 being provided? > > >>>> > > >>> > > >>> No comments on the patch, but ... > > >>> > > >>> Since it required little thought, I ran the test program on next-20220802 and > > >>> was surprised that the issue did not recreate. Even added a simple printk > > >>> to make sure we were getting into vma_wants_writenotify with a hugetlb vma. > > >>> We were. > > >> > > >> > > >> ... does your config have CONFIG_MEM_SOFT_DIRTY enabled? > > >> > > > > > > No, Duh! > > > > > > FYI - Some time back, I started looking at adding soft dirty support for > > > hugetlb mappings. I did not finish that work. But, I seem to recall > > > places where code was operating on hugetlb mappings when perhaps it should > > > not. > > > > > > Perhaps, it would also be good to just disable soft dirty for hugetlb at > > > the source? > > > > I thought about that as well. But I came to the conclusion that without > > patch #2, hugetlb VMAs cannot possibly support write-notify, so there is > > no need to bother in vma_wants_writenotify() at all. > > > > The "root" would be places where we clear VM_SOFTDIRTY. That should only > > be fs/proc/task_mmu.c:clear_refs_write() IIRC. > > > > So I don't particularly care, I consider this patch a bit cleaner and > > more generic, but I can adjust clear_refs_write() instead of there is a > > preference. > > > > After a closer look, I agree that this may be the simplest/cleanest way to > proceed. I was going to suggest that you note hugetlb does not support > softdirty, but see you did in the comment. > > Acked-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Filtering out hugetlbfs in vma_wants_writenotify() is still a bit hard to follow to me, since it's not clear why hugetlbfs never wants writenotify. If it's only about soft-dirty, we could have added the hugetlbfs check into vma_soft_dirty_enabled(), then I think it'll achieve the same thing and much clearer - with the soft-dirty check constantly returning false for it, hugetlbfs shared vmas should have vma_wants_writenotify() naturally return 0 already. For the long term - shouldn't we just enable soft-dirty for hugetlbfs? I remember Mike used to have that in todo. Since we've got patch 2 already, I feel like that's really much close (is the only missing piece the clear refs write part? or maybe some more that I didn't notice). Then patch 1 (or IMHO equivalant check in vma_soft_dirty_enabled(), but maybe in stable trees we don't have vma_soft_dirty_enabled then it's exactly patch 1) can be a stable-only backport just to avoid the bug from triggering. Thanks, -- Peter Xu
