Re: [RFC PATCH RESEND 05/28] mm: add per-VMA lock and helper functions to control it

From: Suren Baghdasaryan
Date: Tue Sep 06 2022 - 13:31:35 EST

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On Tue, Sep 6, 2022 at 6:47 AM Laurent Dufour <ldufour@xxxxxxxxxxxxx> wrote:
>
> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> > Introduce a per-VMA rw_semaphore to be used during page fault handling
> > instead of mmap_lock. Because there are cases when multiple VMAs need
> > to be exclusively locked during VMA tree modifications, instead of the
> > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock
> > exclusively and setting vma->lock_seq to the current mm->lock_seq. When
> > mmap_write_lock holder is done with all modifications and drops mmap_lock,
> > it will increment mm->lock_seq, effectively unlocking all VMAs marked as
> > locked.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Despite a minor comment below,
>
> Reviewed-by: Laurent Dufour <laurent.dufour@xxxxxxxxxx>

Thanks for the reviews Laurent! I'll need some time to double-check
all the VMA locking locations that you spotted as potentially
unnecessary. Admittedly I was a bit paranoid when writing this
patchset and trying not to miss any potential race, so some of them
might indeed be unnecessary. Will reply to each of your comments once
I confirm the need for locking in each case.
Thanks,
Suren.

>
> > ---
> > include/linux/mm.h | 78 +++++++++++++++++++++++++++++++++++++++
> > include/linux/mm_types.h | 7 ++++
> > include/linux/mmap_lock.h | 13 +++++++
> > kernel/fork.c | 4 ++
> > mm/init-mm.c | 3 ++
> > 5 files changed, 105 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 7d322a979455..476bf936c5f0 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -611,6 +611,83 @@ struct vm_operations_struct {
> > unsigned long addr);
> > };
> >
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +static inline void vma_init_lock(struct vm_area_struct *vma)
> > +{
> > + init_rwsem(&vma->lock);
> > + vma->vm_lock_seq = -1;
> > +}
> > +
> > +static inline void vma_mark_locked(struct vm_area_struct *vma)
> > +{
> > + int mm_lock_seq;
> > +
> > + mmap_assert_write_locked(vma->vm_mm);
> > +
> > + /*
> > + * current task is holding mmap_write_lock, both vma->vm_lock_seq and
> > + * mm->mm_lock_seq can't be concurrently modified.
> > + */
> > + mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq);
> > + if (vma->vm_lock_seq == mm_lock_seq)
> > + return;
> > +
> > + down_write(&vma->lock);
> > + vma->vm_lock_seq = mm_lock_seq;
> > + up_write(&vma->lock);
> > +}
> > +
> > +static inline bool vma_read_trylock(struct vm_area_struct *vma)
> > +{
> > + if (unlikely(down_read_trylock(&vma->lock) == 0))
> > + return false;
> > +
> > + /*
> > + * Overflow might produce false locked result but it's not critical.
>
> It might be good to precise here that in the case of false locked, the
> caller is assumed to fallback read locking the mm entirely before doing its
> change relative to that VMA.

Ack.

>
> > + * False unlocked result is critical but is impossible because we
> > + * modify and check vma->vm_lock_seq under vma->lock protection and
> > + * mm->mm_lock_seq modification invalidates all existing locks.
> > + */
> > + if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) {
> > + up_read(&vma->lock);
> > + return false;
> > + }
> > + return true;
> > +}
> > +
> > +static inline void vma_read_unlock(struct vm_area_struct *vma)
> > +{
> > + up_read(&vma->lock);
> > +}
> > +
> > +static inline void vma_assert_locked(struct vm_area_struct *vma)
> > +{
> > + lockdep_assert_held(&vma->lock);
> > + VM_BUG_ON_VMA(!rwsem_is_locked(&vma->lock), vma);
> > +}
> > +
> > +static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos)
> > +{
> > + mmap_assert_write_locked(vma->vm_mm);
> > + /*
> > + * current task is holding mmap_write_lock, both vma->vm_lock_seq and
> > + * mm->mm_lock_seq can't be concurrently modified.
> > + */
> > + VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
> > +}
> > +
> > +#else /* CONFIG_PER_VMA_LOCK */
> > +
> > +static inline void vma_init_lock(struct vm_area_struct *vma) {}
> > +static inline void vma_mark_locked(struct vm_area_struct *vma) {}
> > +static inline bool vma_read_trylock(struct vm_area_struct *vma)
> > + { return false; }
> > +static inline void vma_read_unlock(struct vm_area_struct *vma) {}
> > +static inline void vma_assert_locked(struct vm_area_struct *vma) {}
> > +static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos) {}
> > +
> > +#endif /* CONFIG_PER_VMA_LOCK */
> > +
> > static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> > {
> > static const struct vm_operations_struct dummy_vm_ops = {};
> > @@ -619,6 +696,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> > vma->vm_mm = mm;
> > vma->vm_ops = &dummy_vm_ops;
> > INIT_LIST_HEAD(&vma->anon_vma_chain);
> > + vma_init_lock(vma);
> > }
> >
> > static inline void vma_set_anonymous(struct vm_area_struct *vma)
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index bed25ef7c994..6a03f59c1e78 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -486,6 +486,10 @@ struct vm_area_struct {
> > struct mempolicy *vm_policy; /* NUMA policy for the VMA */
> > #endif
> > struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> > +#ifdef CONFIG_PER_VMA_LOCK
> > + struct rw_semaphore lock;
> > + int vm_lock_seq;
> > +#endif
> > } __randomize_layout;
> >
> > struct kioctx_table;
> > @@ -567,6 +571,9 @@ struct mm_struct {
> > * init_mm.mmlist, and are protected
> > * by mmlist_lock
> > */
> > +#ifdef CONFIG_PER_VMA_LOCK
> > + int mm_lock_seq;
> > +#endif
> >
> >
> > unsigned long hiwater_rss; /* High-watermark of RSS usage */
> > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > index e49ba91bb1f0..a391ae226564 100644
> > --- a/include/linux/mmap_lock.h
> > +++ b/include/linux/mmap_lock.h
> > @@ -72,6 +72,17 @@ static inline void mmap_assert_write_locked(struct mm_struct *mm)
> > VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
> > }
> >
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +static inline void vma_mark_unlocked_all(struct mm_struct *mm)
> > +{
> > + mmap_assert_write_locked(mm);
> > + /* No races during update due to exclusive mmap_lock being held */
> > + WRITE_ONCE(mm->mm_lock_seq, mm->mm_lock_seq + 1);
> > +}
> > +#else
> > +static inline void vma_mark_unlocked_all(struct mm_struct *mm) {}
> > +#endif
> > +
> > static inline void mmap_init_lock(struct mm_struct *mm)
> > {
> > init_rwsem(&mm->mmap_lock);
> > @@ -114,12 +125,14 @@ static inline bool mmap_write_trylock(struct mm_struct *mm)
> > static inline void mmap_write_unlock(struct mm_struct *mm)
> > {
> > __mmap_lock_trace_released(mm, true);
> > + vma_mark_unlocked_all(mm);
> > up_write(&mm->mmap_lock);
> > }
> >
> > static inline void mmap_write_downgrade(struct mm_struct *mm)
> > {
> > __mmap_lock_trace_acquire_returned(mm, false, true);
> > + vma_mark_unlocked_all(mm);
> > downgrade_write(&mm->mmap_lock);
> > }
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 614872438393..bfab31ecd11e 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -475,6 +475,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> > */
> > *new = data_race(*orig);
> > INIT_LIST_HEAD(&new->anon_vma_chain);
> > + vma_init_lock(new);
> > new->vm_next = new->vm_prev = NULL;
> > dup_anon_vma_name(orig, new);
> > }
> > @@ -1130,6 +1131,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> > seqcount_init(&mm->write_protect_seq);
> > mmap_init_lock(mm);
> > INIT_LIST_HEAD(&mm->mmlist);
> > +#ifdef CONFIG_PER_VMA_LOCK
> > + WRITE_ONCE(mm->mm_lock_seq, 0);
> > +#endif
> > mm_pgtables_bytes_init(mm);
> > mm->map_count = 0;
> > mm->locked_vm = 0;
> > diff --git a/mm/init-mm.c b/mm/init-mm.c
> > index fbe7844d0912..8399f90d631c 100644
> > --- a/mm/init-mm.c
> > +++ b/mm/init-mm.c
> > @@ -37,6 +37,9 @@ struct mm_struct init_mm = {
> > .page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
> > .arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
> > .mmlist = LIST_HEAD_INIT(init_mm.mmlist),
> > +#ifdef CONFIG_PER_VMA_LOCK
> > + .mm_lock_seq = 0,
> > +#endif
> > .user_ns = &init_user_ns,
> > .cpu_bitmap = CPU_BITS_NONE,
> > #ifdef CONFIG_IOMMU_SVA
>

Next message: Phil Auld: "Re: [PATCH] drivers/base: Fix unsigned comparison to -1 in CPUMAP_FILE_MAX_BYTES"
Previous message: Roberto Sassu: "[PATCH 2/7] libbpf: Define bpf_get_fd_opts and introduce bpf_map_get_fd_by_id_opts()"
In reply to: Laurent Dufour: "Re: [RFC PATCH RESEND 05/28] mm: add per-VMA lock and helper functions to control it"
Next in thread: Suren Baghdasaryan: "[RFC PATCH RESEND 06/28] mm: mark VMA as locked whenever vma->vm_flags are modified"
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