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Date:   Mon, 11 Jan 2021 18:09:56 +0000
From:   "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To:     Axel Rasmussen <axelrasmussen@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Chinwen Chang <chinwen.chang@mediatek.com>,
        Huang Ying <ying.huang@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Lokesh Gidra <lokeshgidra@google.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Michel Lespinasse <walken@google.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Peter Xu <peterx@redhat.com>, Shaohua Li <shli@fb.com>,
        Shawn Anastasio <shawn@anastas.io>,
        Steven Rostedt <rostedt@goodmis.org>,
        Steven Price <steven.price@arm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
        Adam Ruprecht <ruprecht@google.com>,
        Cannon Matthews <cannonmatthews@google.com>,
        David Rientjes <rientjes@google.com>,
        Oliver Upton <oupton@google.com>
Subject: Re: [RFC PATCH 1/2] userfaultfd: add minor fault registration mode
Message-ID: <20210111180956.GB2966@work-vm>
References: <20210107190453.3051110-1-axelrasmussen@google.com>
 <20210107190453.3051110-2-axelrasmussen@google.com>
 <20210111115818.GG2965@work-vm>
 <CAJHvVciTgttHeD2YXvn0MxJm0Xu8ccbJRRrqwyWcXuswfxnkhw@mail.gmail.com>
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* Axel Rasmussen (axelrasmussen@google.com) wrote:
> On Mon, Jan 11, 2021 at 3:58 AM Dr. David Alan Gilbert
> <dgilbert@redhat.com> wrote:
> >
> > * Axel Rasmussen (axelrasmussen@google.com) wrote:
> > > This feature allows userspace to intercept "minor" faults. By "minor"
> > > fault, I mean the following situation:
> > >
> > > Let there exist two mappings (i.e., VMAs) to the same page(s) (shared
> > > memory). One of the mappings is registered with userfaultfd (in minor
> > > mode), and the other is not. Via the non-UFFD mapping, the underlying
> > > pages have already been allocated & filled with some contents. The UFFD
> > > mapping has not yet been faulted in; when it is touched for the first
> > > time, this results in what I'm calling a "minor" fault. As a concrete
> > > example, when working with hugetlbfs, we have huge_pte_none(), but
> > > find_lock_page() finds an existing page.
> > >
> > > This commit adds the new registration mode, and sets the relevant flag
> > > on the VMAs being registered. In the hugetlb fault path, if we find
> > > that we have huge_pte_none(), but find_lock_page() does indeed find an
> > > existing page, then we have a "minor" fault, and if the VMA has the
> > > userfaultfd registration flag, we call into userfaultfd to handle it.
> > >
> > > Why add a new registration mode, as opposed to adding a feature to
> > > MISSING registration, like UFFD_FEATURE_SIGBUS?
> > >
> > > - The semantics are significantly different. UFFDIO_COPY or
> > >   UFFDIO_ZEROPAGE do not make sense for these minor faults; userspace
> > >   would instead just memset() or memcpy() or whatever via the non-UFFD
> > >   mapping. Unlike MISSING registration, MINOR registration only makes
> > >   sense for shared memory (hugetlbfs or shmem [to be supported in future
> > >   commits]).
> >
> > Is there a reason that UFFDIO_COPY can't work with this and a non-shared
> > mapping? and/or the fallocate or hole punching we currently use?
> 
> It could work, with a small modification. Mainly I just didn't see it
> being useful since memcpy() or similar works in this case. (We don't
> have the atomicity problem UFFDIO_COPY is trying to solve, since we're
> modifying via a non-UFFD VMA.)

I guess I was thinking how easily to modify my existing postcopy code;

> Maybe another (weaker?) argument against it is, if it happily accepts
> VMAs both with and without backing pages, it may be easier to misuse.
> E.g., if you mistakenly give it the wrong address, and it overwrites
> some existing page you didn't mean to modify.
> 
> I don't feel particularly strongly, if it strikes you as cleaner to
> just support it, I'll do so in the next revision.

Not necessarily; I was just thinking still in the way I'm used to with
the existing interface.

Dave

> >
> > Dave
> >
> > > - Doing so would make handle_userfault()'s "reason" argument confusing.
> > >   We'd pass in "MISSING" even if the pages weren't really missing.
> > >
> > > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> > > ---
> > >  fs/proc/task_mmu.c               |  1 +
> > >  fs/userfaultfd.c                 | 80 +++++++++++++++++++-------------
> > >  include/linux/mm.h               |  1 +
> > >  include/linux/userfaultfd_k.h    | 12 ++++-
> > >  include/trace/events/mmflags.h   |  1 +
> > >  include/uapi/linux/userfaultfd.h | 15 +++++-
> > >  mm/hugetlb.c                     | 31 +++++++++++++
> > >  7 files changed, 107 insertions(+), 34 deletions(-)
> > >
> > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > index ee5a235b3056..108faf719a83 100644
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -651,6 +651,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
> > >               [ilog2(VM_MTE)]         = "mt",
> > >               [ilog2(VM_MTE_ALLOWED)] = "",
> > >  #endif
> > > +             [ilog2(VM_UFFD_MINOR)]  = "ui",
> > >  #ifdef CONFIG_ARCH_HAS_PKEYS
> > >               /* These come out via ProtectionKey: */
> > >               [ilog2(VM_PKEY_BIT0)]   = "",
> > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > index 894cc28142e7..0a661422eb19 100644
> > > --- a/fs/userfaultfd.c
> > > +++ b/fs/userfaultfd.c
> > > @@ -28,6 +28,8 @@
> > >  #include <linux/security.h>
> > >  #include <linux/hugetlb.h>
> > >
> > > +#define __VM_UFFD_FLAGS (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR)
> > > +
> > >  int sysctl_unprivileged_userfaultfd __read_mostly;
> > >
> > >  static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
> > > @@ -196,24 +198,21 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
> > >       msg_init(&msg);
> > >       msg.event = UFFD_EVENT_PAGEFAULT;
> > >       msg.arg.pagefault.address = address;
> > > +     /*
> > > +      * These flags indicate why the userfault occurred:
> > > +      * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
> > > +      * - UFFD_PAGEFAULT_FLAG_MINOR indicates a minor fault.
> > > +      * - Neither of these flags being set indicates a MISSING fault.
> > > +      *
> > > +      * Separately, UFFD_PAGEFAULT_FLAG_WRITE indicates it was a write
> > > +      * fault. Otherwise, it was a read fault.
> > > +      */
> > >       if (flags & FAULT_FLAG_WRITE)
> > > -             /*
> > > -              * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
> > > -              * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
> > > -              * was not set in a UFFD_EVENT_PAGEFAULT, it means it
> > > -              * was a read fault, otherwise if set it means it's
> > > -              * a write fault.
> > > -              */
> > >               msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
> > >       if (reason & VM_UFFD_WP)
> > > -             /*
> > > -              * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
> > > -              * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
> > > -              * not set in a UFFD_EVENT_PAGEFAULT, it means it was
> > > -              * a missing fault, otherwise if set it means it's a
> > > -              * write protect fault.
> > > -              */
> > >               msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
> > > +     if (reason & VM_UFFD_MINOR)
> > > +             msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_MINOR;
> > >       if (features & UFFD_FEATURE_THREAD_ID)
> > >               msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
> > >       return msg;
> > > @@ -400,8 +399,10 @@ 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)));
> > >
> > >       if (ctx->features & UFFD_FEATURE_SIGBUS)
> > >               goto out;
> > > @@ -611,7 +612,7 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
> > >               for (vma = mm->mmap; vma; vma = vma->vm_next)
> > >                       if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
> > >                               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> > > -                             vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
> > > +                             vma->vm_flags &= ~__VM_UFFD_FLAGS;
> > >                       }
> > >               mmap_write_unlock(mm);
> > >
> > > @@ -643,7 +644,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> > >       octx = vma->vm_userfaultfd_ctx.ctx;
> > >       if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
> > >               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> > > -             vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
> > > +             vma->vm_flags &= ~__VM_UFFD_FLAGS;
> > >               return 0;
> > >       }
> > >
> > > @@ -725,7 +726,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
> > >       } else {
> > >               /* Drop uffd context if remap feature not enabled */
> > >               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> > > -             vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
> > > +             vma->vm_flags &= ~__VM_UFFD_FLAGS;
> > >       }
> > >  }
> > >
> > > @@ -866,12 +867,12 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
> > >       for (vma = mm->mmap; vma; vma = vma->vm_next) {
> > >               cond_resched();
> > >               BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
> > > -                    !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
> > > +                    !!(vma->vm_flags & __VM_UFFD_FLAGS));
> > >               if (vma->vm_userfaultfd_ctx.ctx != ctx) {
> > >                       prev = vma;
> > >                       continue;
> > >               }
> > > -             new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
> > > +             new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
> > >               prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
> > >                                new_flags, vma->anon_vma,
> > >                                vma->vm_file, vma->vm_pgoff,
> > > @@ -1260,9 +1261,26 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
> > >                                    unsigned long vm_flags)
> > >  {
> > >       /* FIXME: add WP support to hugetlbfs and shmem */
> > > -     return vma_is_anonymous(vma) ||
> > > -             ((is_vm_hugetlb_page(vma) || vma_is_shmem(vma)) &&
> > > -              !(vm_flags & VM_UFFD_WP));
> > > +     if (vm_flags & VM_UFFD_WP) {
> > > +             if (is_vm_hugetlb_page(vma) || vma_is_shmem(vma))
> > > +                     return false;
> > > +     }
> > > +
> > > +     if (vm_flags & VM_UFFD_MINOR) {
> > > +             /*
> > > +              * The use case for minor registration (intercepting minor
> > > +              * faults) is to handle the case where a page is present, but
> > > +              * needs to be modified before it can be used. This requires
> > > +              * two mappings: one with UFFD registration, and one without.
> > > +              * So, it only makes sense to do this with shared memory.
> > > +              */
> > > +             /* FIXME: Add minor fault interception for shmem. */
> > > +             if (!(is_vm_hugetlb_page(vma) && (vma->vm_flags & VM_SHARED)))
> > > +                     return false;
> > > +     }
> > > +
> > > +     return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
> > > +            vma_is_shmem(vma);
> > >  }
> > >
> > >  static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > > @@ -1288,14 +1306,15 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > >       ret = -EINVAL;
> > >       if (!uffdio_register.mode)
> > >               goto out;
> > > -     if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
> > > -                                  UFFDIO_REGISTER_MODE_WP))
> > > +     if (uffdio_register.mode & ~UFFD_API_REGISTER_MODES)
> > >               goto out;
> > >       vm_flags = 0;
> > >       if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
> > >               vm_flags |= VM_UFFD_MISSING;
> > >       if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP)
> > >               vm_flags |= VM_UFFD_WP;
> > > +     if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR)
> > > +             vm_flags |= VM_UFFD_MINOR;
> > >
> > >       ret = validate_range(mm, &uffdio_register.range.start,
> > >                            uffdio_register.range.len);
> > > @@ -1339,7 +1358,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > >               cond_resched();
> > >
> > >               BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
> > > -                    !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
> > > +                    !!(cur->vm_flags & __VM_UFFD_FLAGS));
> > >
> > >               /* check not compatible vmas */
> > >               ret = -EINVAL;
> > > @@ -1419,8 +1438,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > >                       start = vma->vm_start;
> > >               vma_end = min(end, vma->vm_end);
> > >
> > > -             new_flags = (vma->vm_flags &
> > > -                          ~(VM_UFFD_MISSING|VM_UFFD_WP)) | vm_flags;
> > > +             new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags;
> > >               prev = vma_merge(mm, prev, start, vma_end, new_flags,
> > >                                vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> > >                                vma_policy(vma),
> > > @@ -1539,7 +1557,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> > >               cond_resched();
> > >
> > >               BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
> > > -                    !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
> > > +                    !!(cur->vm_flags & __VM_UFFD_FLAGS));
> > >
> > >               /*
> > >                * Check not compatible vmas, not strictly required
> > > @@ -1590,7 +1608,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> > >                       wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
> > >               }
> > >
> > > -             new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
> > > +             new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
> > >               prev = vma_merge(mm, prev, start, vma_end, new_flags,
> > >                                vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> > >                                vma_policy(vma),
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index ecdf8a8cd6ae..1d7041bd3148 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -276,6 +276,7 @@ extern unsigned int kobjsize(const void *objp);
> > >  #define VM_PFNMAP    0x00000400      /* Page-ranges managed without "struct page", just pure PFN */
> > >  #define VM_DENYWRITE 0x00000800      /* ETXTBSY on write attempts.. */
> > >  #define VM_UFFD_WP   0x00001000      /* wrprotect pages tracking */
> > > +#define VM_UFFD_MINOR        0x00002000      /* minor fault interception */
> > >
> > >  #define VM_LOCKED    0x00002000
> > >  #define VM_IO           0x00004000   /* Memory mapped I/O or similar */
> > > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > > index a8e5f3ea9bb2..0c8c5fa5efc8 100644
> > > --- a/include/linux/userfaultfd_k.h
> > > +++ b/include/linux/userfaultfd_k.h
> > > @@ -62,6 +62,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
> > >       return vma->vm_flags & VM_UFFD_WP;
> > >  }
> > >
> > > +static inline bool userfaultfd_minor(struct vm_area_struct *vma)
> > > +{
> > > +     return vma->vm_flags & VM_UFFD_MINOR;
> > > +}
> > > +
> > >  static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
> > >                                     pte_t pte)
> > >  {
> > > @@ -76,7 +81,7 @@ static inline bool userfaultfd_huge_pmd_wp(struct vm_area_struct *vma,
> > >
> > >  static inline bool userfaultfd_armed(struct vm_area_struct *vma)
> > >  {
> > > -     return vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP);
> > > +     return vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR);
> > >  }
> > >
> > >  extern int dup_userfaultfd(struct vm_area_struct *, struct list_head *);
> > > @@ -123,6 +128,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
> > >       return false;
> > >  }
> > >
> > > +static inline bool userfaultfd_minor(struct vm_area_struct *vma)
> > > +{
> > > +     return false;
> > > +}
> > > +
> > >  static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
> > >                                     pte_t pte)
> > >  {
> > > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> > > index 67018d367b9f..2d583ffd4100 100644
> > > --- a/include/trace/events/mmflags.h
> > > +++ b/include/trace/events/mmflags.h
> > > @@ -151,6 +151,7 @@ IF_HAVE_PG_ARCH_2(PG_arch_2,              "arch_2"        )
> > >       {VM_PFNMAP,                     "pfnmap"        },              \
> > >       {VM_DENYWRITE,                  "denywrite"     },              \
> > >       {VM_UFFD_WP,                    "uffd_wp"       },              \
> > > +     {VM_UFFD_MINOR,                 "uffd_minor"    },              \
> > >       {VM_LOCKED,                     "locked"        },              \
> > >       {VM_IO,                         "io"            },              \
> > >       {VM_SEQ_READ,                   "seqread"       },              \
> > > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > > index 5f2d88212f7c..1cc2cd8a5279 100644
> > > --- a/include/uapi/linux/userfaultfd.h
> > > +++ b/include/uapi/linux/userfaultfd.h
> > > @@ -19,15 +19,19 @@
> > >   * means the userland is reading).
> > >   */
> > >  #define UFFD_API ((__u64)0xAA)
> > > +#define UFFD_API_REGISTER_MODES (UFFDIO_REGISTER_MODE_MISSING |      \
> > > +                              UFFDIO_REGISTER_MODE_WP |      \
> > > +                              UFFDIO_REGISTER_MODE_MINOR)
> > >  #define UFFD_API_FEATURES (UFFD_FEATURE_PAGEFAULT_FLAG_WP |  \
> > >                          UFFD_FEATURE_EVENT_FORK |            \
> > >                          UFFD_FEATURE_EVENT_REMAP |           \
> > > -                        UFFD_FEATURE_EVENT_REMOVE |  \
> > > +                        UFFD_FEATURE_EVENT_REMOVE |          \
> > >                          UFFD_FEATURE_EVENT_UNMAP |           \
> > >                          UFFD_FEATURE_MISSING_HUGETLBFS |     \
> > >                          UFFD_FEATURE_MISSING_SHMEM |         \
> > >                          UFFD_FEATURE_SIGBUS |                \
> > > -                        UFFD_FEATURE_THREAD_ID)
> > > +                        UFFD_FEATURE_THREAD_ID |             \
> > > +                        UFFD_FEATURE_MINOR_FAULT_HUGETLBFS)
> > >  #define UFFD_API_IOCTLS                              \
> > >       ((__u64)1 << _UFFDIO_REGISTER |         \
> > >        (__u64)1 << _UFFDIO_UNREGISTER |       \
> > > @@ -127,6 +131,7 @@ struct uffd_msg {
> > >  /* flags for UFFD_EVENT_PAGEFAULT */
> > >  #define UFFD_PAGEFAULT_FLAG_WRITE    (1<<0)  /* If this was a write fault */
> > >  #define UFFD_PAGEFAULT_FLAG_WP               (1<<1)  /* If reason is VM_UFFD_WP */
> > > +#define UFFD_PAGEFAULT_FLAG_MINOR    (1<<2)  /* If reason is VM_UFFD_MINOR */
> > >
> > >  struct uffdio_api {
> > >       /* userland asks for an API number and the features to enable */
> > > @@ -171,6 +176,10 @@ struct uffdio_api {
> > >        *
> > >        * UFFD_FEATURE_THREAD_ID pid of the page faulted task_struct will
> > >        * be returned, if feature is not requested 0 will be returned.
> > > +      *
> > > +      * UFFD_FEATURE_MINOR_FAULT_HUGETLBFS indicates that minor faults
> > > +      * can be intercepted (via REGISTER_MODE_MINOR) for
> > > +      * hugetlbfs-backed pages.
> > >        */
> > >  #define UFFD_FEATURE_PAGEFAULT_FLAG_WP               (1<<0)
> > >  #define UFFD_FEATURE_EVENT_FORK                      (1<<1)
> > > @@ -181,6 +190,7 @@ struct uffdio_api {
> > >  #define UFFD_FEATURE_EVENT_UNMAP             (1<<6)
> > >  #define UFFD_FEATURE_SIGBUS                  (1<<7)
> > >  #define UFFD_FEATURE_THREAD_ID                       (1<<8)
> > > +#define UFFD_FEATURE_MINOR_FAULT_HUGETLBFS   (1<<9)
> > >       __u64 features;
> > >
> > >       __u64 ioctls;
> > > @@ -195,6 +205,7 @@ struct uffdio_register {
> > >       struct uffdio_range range;
> > >  #define UFFDIO_REGISTER_MODE_MISSING ((__u64)1<<0)
> > >  #define UFFDIO_REGISTER_MODE_WP              ((__u64)1<<1)
> > > +#define UFFDIO_REGISTER_MODE_MINOR   ((__u64)1<<2)
> > >       __u64 mode;
> > >
> > >       /*
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index a2602969873d..0ba8f2f5a4ae 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -4377,6 +4377,37 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
> > >               }
> > >       }
> > >
> > > +     /* Check for page in userfault range. */
> > > +     if (!new_page && 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;
> > > +     }
> > > +
> > >       /*
> > >        * If we are going to COW a private mapping later, we examine the
> > >        * pending reservations for this page now. This will ensure that
> > > --
> > > 2.29.2.729.g45daf8777d-goog
> > >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK