On Wed, Jun 28, 2023 at 6:58 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Wed, Jun 28, 2023 at 12:18:00AM -0700, Suren Baghdasaryan wrote:
> > Enable handle_userfault to operate under VMA lock by releasing VMA lock
> > instead of mmap_lock and retrying. Note that FAULT_FLAG_RETRY_NOWAIT
> > should never be used when handling faults under per-VMA lock protection
> > because that would break the assumption that lock is dropped on retry.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>
> Besides the NOWAIT typo all look sane. Since there seems to need at least
> one more version I'll still comment on a few things..
>
> > ---
> > fs/userfaultfd.c | 39 ++++++++++++++++++---------------------
> > include/linux/mm.h | 39 +++++++++++++++++++++++++++++++++++++++
> > mm/filemap.c | 8 --------
> > mm/memory.c | 9 ---------
> > 4 files changed, 57 insertions(+), 38 deletions(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 4e800bb7d2ab..d019e7df6f15 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -277,17 +277,16 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
> > * hugepmd ranges.
> > */
> > static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > - struct vm_area_struct *vma,
> > - unsigned long address,
> > - unsigned long flags,
> > - unsigned long reason)
> > + struct vm_fault *vmf,
> > + unsigned long reason)
> > {
> > + struct vm_area_struct *vma = vmf->vma;
> > pte_t *ptep, pte;
> > bool ret = true;
> >
> > - mmap_assert_locked(ctx->mm);
> > + assert_fault_locked(ctx->mm, vmf);
>
> AFAIU ctx->mm must be the same as vma->vm_mm here, so maybe we can also
> drop *ctx here altogether if we've already dropped plenty.
Ack. I was not sure if the ctx->mm would always be the same as vmf->mm.
>
> >
> > - ptep = hugetlb_walk(vma, address, vma_mmu_pagesize(vma));
> > + ptep = hugetlb_walk(vma, vmf->address, vma_mmu_pagesize(vma));
> > if (!ptep)
> > goto out;
> >
> > @@ -308,10 +307,8 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > }
> > #else
> > static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > - struct vm_area_struct *vma,
> > - unsigned long address,
> > - unsigned long flags,
> > - unsigned long reason)
> > + struct vm_fault *vmf,
> > + unsigned long reason)
> > {
> > return false; /* should never get here */
> > }
> > @@ -325,11 +322,11 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > * threads.
> > */
> > static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> > - unsigned long address,
> > - unsigned long flags,
> > + struct vm_fault *vmf,
> > unsigned long reason)
> > {
> > struct mm_struct *mm = ctx->mm;
> > + unsigned long address = vmf->address;
> > pgd_t *pgd;
> > p4d_t *p4d;
> > pud_t *pud;
> > @@ -337,7 +334,7 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> > pte_t *pte;
> > bool ret = true;
> >
> > - mmap_assert_locked(mm);
> > + assert_fault_locked(mm, vmf);
> >
> > pgd = pgd_offset(mm, address);
> > if (!pgd_present(*pgd))
> > @@ -445,7 +442,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> > * Coredumping runs without mmap_lock so we can only check that
> > * the mmap_lock is held, if PF_DUMPCORE was not set.
> > */
> > - mmap_assert_locked(mm);
> > + assert_fault_locked(mm, vmf);
> >
> > ctx = vma->vm_userfaultfd_ctx.ctx;
> > if (!ctx)
> > @@ -522,8 +519,11 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> > * and wait.
> > */
> > ret = VM_FAULT_RETRY;
> > - if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
> > + if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT) {
> > + /* Per-VMA lock is expected to be dropped on VM_FAULT_RETRY */
> > + BUG_ON(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
>
> Here is not the only place that we can have FAULT_FLAG_RETRY_NOWAIT.
> E.g. folio_lock_or_retry() can also get it, so check here may or may not
> help much.
>
> The other thing is please consider not using BUG_ON if possible.
> WARN_ON_ONCE() is IMHO always more preferred if the kernel can still try to
> run even if it triggers.
Ack.
>
> I'd rather drop this change, leaving space for future when vma lock may be
> supported in gup paths with NOWAIT, then here it'll work naturally, afaiu.
> If we really want a sanity check, maybe the best place is when entering
> handle_mm_fault(), to be explicit, sanitize_fault_flags().
That sounds like a good idea. Thanks!
>
> > goto out;
> > + }
> >
> > /* take the reference before dropping the mmap_lock */
> > userfaultfd_ctx_get(ctx);
> > @@ -561,15 +561,12 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> > spin_unlock_irq(&ctx->fault_pending_wqh.lock);
> >
> > if (!is_vm_hugetlb_page(vma))
> > - must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
> > - reason);
> > + must_wait = userfaultfd_must_wait(ctx, vmf, reason);
> > else
> > - must_wait = userfaultfd_huge_must_wait(ctx, vma,
> > - vmf->address,
> > - vmf->flags, reason);
> > + must_wait = userfaultfd_huge_must_wait(ctx, vmf, reason);
> > if (is_vm_hugetlb_page(vma))
> > hugetlb_vma_unlock_read(vma);
> > - mmap_read_unlock(mm);
> > + release_fault_lock(vmf);
> >
> > if (likely(must_wait && !READ_ONCE(ctx->released))) {
> > wake_up_poll(&ctx->fd_wqh, EPOLLIN);
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index fec149585985..70bb2f923e33 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -705,6 +705,17 @@ static inline bool vma_try_start_write(struct vm_area_struct *vma)
> > return true;
> > }
> >
> > +static inline void vma_assert_locked(struct vm_area_struct *vma)
> > +{
> > + int mm_lock_seq;
> > +
> > + if (__is_vma_write_locked(vma, &mm_lock_seq))
> > + return;
> > +
> > + lockdep_assert_held(&vma->vm_lock->lock);
> > + VM_BUG_ON_VMA(!rwsem_is_locked(&vma->vm_lock->lock), vma);
> > +}
> > +
> > static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> > {
> > int mm_lock_seq;
> > @@ -723,6 +734,23 @@ static inline void vma_mark_detached(struct vm_area_struct *vma, bool detached)
> > struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> > unsigned long address);
> >
> > +static inline
> > +void assert_fault_locked(struct mm_struct *mm, struct vm_fault *vmf)
> > +{
> > + if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > + vma_assert_locked(vmf->vma);
> > + else
> > + mmap_assert_locked(mm);
> > +}
> > +
> > +static inline void release_fault_lock(struct vm_fault *vmf)
> > +{
> > + if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > + vma_end_read(vmf->vma);
> > + else
> > + mmap_read_unlock(vmf->vma->vm_mm);
> > +}
> > +
> > #else /* CONFIG_PER_VMA_LOCK */
> >
> > static inline void vma_init_lock(struct vm_area_struct *vma) {}
> > @@ -736,6 +764,17 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma) {}
> > static inline void vma_mark_detached(struct vm_area_struct *vma,
> > bool detached) {}
> >
> > +static inline
> > +void assert_fault_locked(struct mm_struct *mm, struct vm_fault *vmf)
> > +{
> > + mmap_assert_locked(mm);
> > +}
> > +
> > +static inline void release_fault_lock(struct vm_fault *vmf)
> > +{
> > + mmap_read_unlock(vmf->vma->vm_mm);
> > +}
> > +
> > #endif /* CONFIG_PER_VMA_LOCK */
> >
> > /*
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 7ee078e1a0d2..d4d8f474e0c5 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1699,14 +1699,6 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
> > return ret;
> > }
> >
> > -static void release_fault_lock(struct vm_fault *vmf)
> > -{
> > - if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > - vma_end_read(vmf->vma);
> > - else
> > - mmap_read_unlock(vmf->vma->vm_mm);
> > -}
>
> The movement is fine but may not be the cleanest. It'll be nicer to me if
> it's put at the right place when introduced - after all in the same series.
Ack.
Thanks for the review!
>
> Thanks,
>
> > -
> > /*
> > * Return values:
> > * 0 - folio is locked.
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 76c7907e7286..c6c759922f39 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5294,15 +5294,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> > if (!vma_start_read(vma))
> > goto inval;
> >
> > - /*
> > - * Due to the possibility of userfault handler dropping mmap_lock, avoid
> > - * it for now and fall back to page fault handling under mmap_lock.
> > - */
> > - if (userfaultfd_armed(vma)) {
> > - vma_end_read(vma);
> > - goto inval;
> > - }
> > -
> > /* Check since vm_start/vm_end might change before we lock the VMA */
> > if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> > vma_end_read(vma);
> > --
> > 2.41.0.162.gfafddb0af9-goog
> >
>
> --
> Peter Xu
>
