Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode

[Axel Rasmussen]

On Wed, Feb 24, 2021 at 4:26 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 2/18/21 4:48 PM, Axel Rasmussen wrote:
> <snip>
> > @@ -401,8 +398,10 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >
> >       BUG_ON(ctx->mm != mm);
> >
> > -     VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
> > -     VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
> > +     /* Any unrecognized flag is a bug. */
> > +     VM_BUG_ON(reason & ~__VM_UFFD_FLAGS);
> > +     /* 0 or > 1 flags set is a bug; we expect exactly 1. */
> > +     VM_BUG_ON(!reason || !!(reason & (reason - 1)));
>
> I may be confused, but that seems to be checking for a flag value of 1
> as opposed to one flag being set?

(Assuming I implemented it correctly!) It's the logical negation of
this trick: https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2
So, it's "VM_BUG_ON(reason is *not* a power of 2)".

Maybe the double negation makes it overly confusing? It ought to be
equivalent if we remove it and just say:
VM_BUG_ON(!reason || (reason & (reason - 1)));

>
> >
> >       if (ctx->features & UFFD_FEATURE_SIGBUS)
> >               goto out;
> <snip>
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 3bfba75f6cbd..0388107da4b1 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -4352,6 +4352,38 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
> >                               VM_FAULT_SET_HINDEX(hstate_index(h));
> >                       goto backout_unlocked;
> >               }
> > +
> > +             /* Check for page in userfault range. */
> > +             if (userfaultfd_minor(vma)) {
> > +                     u32 hash;
> > +                     struct vm_fault vmf = {
> > +                             .vma = vma,
> > +                             .address = haddr,
> > +                             .flags = flags,
> > +                             /*
> > +                              * Hard to debug if it ends up being used by a
> > +                              * callee that assumes something about the
> > +                              * other uninitialized fields... same as in
> > +                              * memory.c
> > +                              */
> > +                     };
> > +
> > +                     unlock_page(page);
> > +
> > +                     /*
> > +                      * hugetlb_fault_mutex and i_mmap_rwsem must be dropped
> > +                      * before handling userfault.  Reacquire after handling
> > +                      * fault to make calling code simpler.
> > +                      */
> > +
> > +                     hash = hugetlb_fault_mutex_hash(mapping, idx);
> > +                     mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> > +                     i_mmap_unlock_read(mapping);
> > +                     ret = handle_userfault(&vmf, VM_UFFD_MINOR);
> > +                     i_mmap_lock_read(mapping);
> > +                     mutex_lock(&hugetlb_fault_mutex_table[hash]);
> > +                     goto out;
> > +             }
> >       }
> >
> >       /*
> >
>
> I'm good with the hugetlb.c changes.  Since this in nearly identical to
> the other handle_userfault() in this routine, it might be good to create
> a common wrapper.  But, that is not required.

Makes sense, I can send a v9 with a helper for this defined. I'll wait
until at least next week to do so, to pick up any other comments v8
may get in the meantime.

Thanks!

> --
> Mike Kravetz