On 08/05/22 19:13, Peter Xu wrote: > 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. It looks like vma_soft_dirty_enabled is recent and not in any stable trees (or even 5.19). Yes, I did start working on hugetlb softdirty support in the past. https://lore.kernel.org/lkml/20210211000322.159437-1-mike.kravetz@xxxxxxxxxx/ Unfortunately, it got preempted by other things. I will try to move it up the priority list. -- Mike Kravetz
