* Axel Rasmussen (axelrasmussen@xxxxxxxxxx) wrote:
> On Mon, Jan 11, 2021 at 3:58 AM Dr. David Alan Gilbert
> <dgilbert@xxxxxxxxxx> wrote:
> >
> > * Axel Rasmussen (axelrasmussen@xxxxxxxxxx) 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@xxxxxxxxxx>
> > > ---
> > > 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@xxxxxxxxxx / Manchester, UK
> >
>
--
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
