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    SubjectRe: [PATCH v1 1/2] mm/hugetlb: fix hugetlb not supporting write-notify
    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@oracle.com>

    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

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