Re: [PATCH v3 3/3] fs/proc/task_mmu: execute PROCMAP_QUERY ioctl under per-vma locks

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Aug 6, 2025 at 12:03 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Wed, Aug 06, 2025 at 08:59:04AM -0700, Suren Baghdasaryan wrote:
> > Utilize per-vma locks to stabilize vma after lookup without taking
> > mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
> > contended, we fall back to mmap_lock but take it only momentarily
> > to lock the vma and release the mmap_lock. In a very unlikely case
> > of vm_refcnt overflow, this fall back path will fail and ioctl is
> > done under mmap_lock protection.
> >
> > This change is designed to reduce mmap_lock contention and prevent
> > PROCMAP_QUERY ioctl calls from blocking address space updates.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> A lot of nits but nothing's really standing out as broken, AFAICT...
>
> > ---
> >  fs/proc/task_mmu.c | 84 +++++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 68 insertions(+), 16 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 45134335e086..0396315dfaee 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -517,28 +517,81 @@ static int pid_maps_open(struct inode *inode, struct file *file)
> >               PROCMAP_QUERY_VMA_FLAGS                         \
> >  )
> >
> > -static int query_vma_setup(struct mm_struct *mm)
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +
> > +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> >  {
> > -     return mmap_read_lock_killable(mm);
> > +     lock_ctx->locked_vma = NULL;
> > +     lock_ctx->mmap_locked = false;
>
> We also do this in lock_vma_range(), seems sensible to factor out? E.g.:
>
> static void ctx_clear_vma(struct proc_maps_locking_ctx *lock_ctx)

That name really confused me :) Maybe lock_vma_ctx_init() or something
along these lines. If we can't think of a good name I would prefer to
keep it as is, given it's only two lines and used only in two places.

> {
>         lock_ctx->locked_vma = NULL;
>         lock_ctx->mmap_locked = false;
> }
>
> > +
> > +     return 0;
> >  }
> >
> > -static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
> > +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> >  {
> > -     mmap_read_unlock(mm);
> > +     if (lock_ctx->mmap_locked)
> > +             mmap_read_unlock(lock_ctx->mm);
>
> Maybe worth a comment as to why we leave lock_ctx->mmap_locked set here?

Sure. The reason is that this is a teardown stage and lock_ctx won't
be used anymore. I guess I could reset it just to leave lock_ctx
consistent instead of adding a comment. Will do that.

>
> > +     else
> > +             unlock_vma(lock_ctx);
>
> Should have said on 2/3, but I wonder if we should prefix with ctx_, as
> 'unlock_vma()' and 'lock_vma()' seem like core functions... esp. since we
> have vma_start_read/write() rather than functions that reference locking.
>
> So - ctx_unlock_vma() and ctx_lock_vma() or unlock_ctx_vma() /
> lock_ctx_vma()?

unlock_ctx_vma() / lock_ctx_vma() sounds good to me.

>
> > +}
> > +
> > +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> > +                                                  unsigned long addr)
> > +{
> > +     struct vm_area_struct *vma;
> > +     struct vma_iterator vmi;
> > +
> > +     if (lock_ctx->mmap_locked)
> > +             return find_vma(lock_ctx->mm, addr);
> > +
> > +     unlock_vma(lock_ctx);
> > +     rcu_read_lock();
> > +     vma_iter_init(&vmi, lock_ctx->mm, addr);
> > +     vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
> > +     rcu_read_unlock();
>
> I think a comment at the top of this block would be useful, something like
> 'We unlock any previously locked VMA and find the next under RCU'.

Ack.

>
> > +
> > +     if (!IS_ERR_OR_NULL(vma)) {
>
> Is the NULL bit here really necessary? presumably lock_ctx->locked_vma is
> expected to be NULL already, so we're not overwriting anything here.
>
> In fact we could get rid of the horrid if/else here with a guard clause like:
>
>         if (!IS_ERR(vma) || PTR_ERR(vma) != -EAGAIN)
>                 return vma;

We still need to assign lock_ctx->locked_vma when !IS_ERR(vma) before
we return the vma, so the lines about would not be correct. I can
change it to:

if (!vma)
    return NULL;

if (!IS_ERR(vma)) {
    lock_ctx->locked_vma = vma;
    return vma;
}

if (PTR_ERR(vma) == -EAGAIN) {
    /* Fallback to mmap_lock on vma->vm_refcnt overflow */
    ...
}
return vma;

I think that would be the equivalent of what I currently have. Would
you prefer that?

>
> (the !IS_ERR() bit is probably a bit redundant but makes things clearer
> vs. just the PTR_ERR() thing)
>
> Then do the rest below.
>
>
> > +             lock_ctx->locked_vma = vma;
> > +     } else if (PTR_ERR(vma) == -EAGAIN) {
> > +             /* Fallback to mmap_lock on vma->vm_refcnt overflow */
> > +             mmap_read_lock(lock_ctx->mm);
> > +             vma = find_vma(lock_ctx->mm, addr);
> > +             lock_ctx->mmap_locked = true;
>
> Kinda sucks we have two separate ways of doing fallback now, this
> open-coded thing and fallback_to_mmap_lock().
>
> Sort of hard to combine since we have subtly diffrent semantics - the RCU
> read lock is being held in the /proc/$pid/maps case, but here we've
> released it already.

Yeah, plus that one uses iterators and this one doesn't... I don't
think it's worth trying to shoehorn them together given that the code
is quite short.

>
> > +     }
> > +
> > +     return vma;
> > +}
> > +
> > +#else /* CONFIG_PER_VMA_LOCK */
> > +
> > +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> > +{
> > +     return mmap_read_lock_killable(lock_ctx->mm);
> >  }
> >
> > -static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
> > +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> >  {
> > -     return find_vma(mm, addr);
> > +     mmap_read_unlock(lock_ctx->mm);
> >  }
> >
> > -static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> > +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> > +                                                  unsigned long addr)
> > +{
> > +     return find_vma(lock_ctx->mm, addr);
> > +}
> > +
> > +#endif  /* CONFIG_PER_VMA_LOCK */
> > +
> > +static struct vm_area_struct *query_matching_vma(struct proc_maps_locking_ctx *lock_ctx,
> >                                                unsigned long addr, u32 flags)
> >  {
> >       struct vm_area_struct *vma;
> >
> >  next_vma:
> > -     vma = query_vma_find_by_addr(mm, addr);
> > +     vma = query_vma_find_by_addr(lock_ctx, addr);
> > +     if (IS_ERR(vma))
> > +             return vma;
> > +
> >       if (!vma)
> >               goto no_vma;
> >
> > @@ -579,11 +632,11 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> >       return ERR_PTR(-ENOENT);
> >  }
> >
> > -static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> > +static int do_procmap_query(struct mm_struct *mm, void __user *uarg)
> >  {
> > +     struct proc_maps_locking_ctx lock_ctx = { .mm = mm };
>
> >       struct procmap_query karg;
> >       struct vm_area_struct *vma;
> > -     struct mm_struct *mm;
> >       const char *name = NULL;
> >       char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
> >       __u64 usize;
> > @@ -610,17 +663,16 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> >       if (!!karg.build_id_size != !!karg.build_id_addr)
> >               return -EINVAL;
> >
> > -     mm = priv->lock_ctx.mm;
> >       if (!mm || !mmget_not_zero(mm))
> >               return -ESRCH;
> >
> > -     err = query_vma_setup(mm);
> > +     err = query_vma_setup(&lock_ctx);
> >       if (err) {
> >               mmput(mm);
> >               return err;
> >       }
> >
> > -     vma = query_matching_vma(mm, karg.query_addr, karg.query_flags);
> > +     vma = query_matching_vma(&lock_ctx, karg.query_addr, karg.query_flags);
> >       if (IS_ERR(vma)) {
> >               err = PTR_ERR(vma);
> >               vma = NULL;
> > @@ -705,7 +757,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> >       }
> >
> >       /* unlock vma or mmap_lock, and put mm_struct before copying data to user */
> > -     query_vma_teardown(mm, vma);
> > +     query_vma_teardown(&lock_ctx);
> >       mmput(mm);
> >
> >       if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
> > @@ -725,7 +777,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> >       return 0;
> >
> >  out:
> > -     query_vma_teardown(mm, vma);
> > +     query_vma_teardown(&lock_ctx);
> >       mmput(mm);
> >       kfree(name_buf);
> >       return err;
> > @@ -738,7 +790,7 @@ static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned l
> >
> >       switch (cmd) {
> >       case PROCMAP_QUERY:
> > -             return do_procmap_query(priv, (void __user *)arg);
> > +             return do_procmap_query(priv->lock_ctx.mm, (void __user *)arg);
>
> OK this confused me until I worked it through.
>
> We set priv->lock_ctx.mm in:
>
> pid_maps_open() -> do_maps_open() -> proc_maps_open()
>
> Which it gets from proc_mem_open() which figures out the mm.
>
> Maybe one for 2/3, but it'd be nice to have a comment saying something
> about how this is set, since it being part of lock_ctx makes it seem like
> it's something that would be set elsewhere.
>
> Since we have fallback stuff and want to thread through this new lokc
> context type I guess it makes sense to put it here but given that's the
> case, let's maybe just add a comment here to clarify.

Ok, something like "lock_ctx.mm is set during file open operation" ?

>
> >       default:
> >               return -ENOIOCTLCMD;
> >       }
> > --
> > 2.50.1.565.gc32cd1483b-goog
> >
>
> Cheers, Lorenzo