Re: [PATCH v3 4/9] fs/procfs: use per-VMA RCU-protected locking in PROCMAP_QUERY API

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Jun 6, 2024 at 10:15 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>
> * Andrii Nakryiko <andrii.nakryiko@gmail.com> [240606 12:52]:
> > On Wed, Jun 5, 2024 at 4:16 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > On Tue, Jun 4, 2024 at 5:25 PM Andrii Nakryiko <andrii@kernel.org> wrote:
> > > >
> > > > Attempt to use RCU-protected per-VMA lock when looking up requested VMA
> > > > as much as possible, only falling back to mmap_lock if per-VMA lock
> > > > failed. This is done so that querying of VMAs doesn't interfere with
> > > > other critical tasks, like page fault handling.
> > > >
> > > > This has been suggested by mm folks, and we make use of a newly added
> > > > internal API that works like find_vma(), but tries to use per-VMA lock.
> > > >
> > > > We have two sets of setup/query/teardown helper functions with different
> > > > implementations depending on availability of per-VMA lock (conditioned
> > > > on CONFIG_PER_VMA_LOCK) to abstract per-VMA lock subtleties.
> > > >
> > > > When per-VMA lock is available, lookup is done under RCU, attempting to
> > > > take a per-VMA lock. If that fails, we fallback to mmap_lock, but then
> > > > proceed to unconditionally grab per-VMA lock again, dropping mmap_lock
> > > > immediately. In this configuration mmap_lock is never helf for long,
> > > > minimizing disruptions while querying.
> > > >
> > > > When per-VMA lock is compiled out, we take mmap_lock once, query VMAs
> > > > using find_vma() API, and then unlock mmap_lock at the very end once as
> > > > well. In this setup we avoid locking/unlocking mmap_lock on every looked
> > > > up VMA (depending on query parameters we might need to iterate a few of
> > > > them).
> > > >
> > > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > > > ---
> > > >  fs/proc/task_mmu.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 46 insertions(+)
> > > >
> > > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > > index 614fbe5d0667..140032ffc551 100644
> > > > --- a/fs/proc/task_mmu.c
> > > > +++ b/fs/proc/task_mmu.c
> > > > @@ -388,6 +388,49 @@ static int pid_maps_open(struct inode *inode, struct file *file)
> > > >                 PROCMAP_QUERY_VMA_FLAGS                         \
> > > >  )
> > > >
> > > > +#ifdef CONFIG_PER_VMA_LOCK
> > > > +static int query_vma_setup(struct mm_struct *mm)
> > > > +{
> > > > +       /* in the presence of per-VMA lock we don't need any setup/teardown */
> > > > +       return 0;
> > > > +}
> > > > +
> > > > +static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
> > > > +{
> > > > +       /* in the presence of per-VMA lock we need to unlock vma, if present */
> > > > +       if (vma)
> > > > +               vma_end_read(vma);
> > > > +}
> > > > +
> > > > +static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
> > > > +{
> > > > +       struct vm_area_struct *vma;
> > > > +
> > > > +       /* try to use less disruptive per-VMA lock */
> > > > +       vma = find_and_lock_vma_rcu(mm, addr);
> > > > +       if (IS_ERR(vma)) {
> > > > +               /* failed to take per-VMA lock, fallback to mmap_lock */
> > > > +               if (mmap_read_lock_killable(mm))
> > > > +                       return ERR_PTR(-EINTR);
> > > > +
> > > > +               vma = find_vma(mm, addr);
> > > > +               if (vma) {
> > > > +                       /*
> > > > +                        * We cannot use vma_start_read() as it may fail due to
> > > > +                        * false locked (see comment in vma_start_read()). We
> > > > +                        * can avoid that by directly locking vm_lock under
> > > > +                        * mmap_lock, which guarantees that nobody can lock the
> > > > +                        * vma for write (vma_start_write()) under us.
> > > > +                        */
> > > > +                       down_read(&vma->vm_lock->lock);
> > >
> > > Hi Andrii,
> > > The above pattern of locking VMA under mmap_lock and then dropping
> > > mmap_lock is becoming more common. Matthew had an RFC proposal for an
> > > API to do this here:
> > > https://lore.kernel.org/all/ZivhG0yrbpFqORDw@casper.infradead.org/. It
> > > might be worth reviving that discussion.
> >
> > Sure, it would be nice to have generic and blessed primitives to use
> > here. But the good news is that once this is all figured out by you mm
> > folks, it should be easy to make use of those primitives here, right?
> >
> > >
> > > > +               }
> > > > +
> > > > +               mmap_read_unlock(mm);
> > >
> > > Later on in your code you are calling get_vma_name() which might call
> > > anon_vma_name() to retrieve user-defined VMA name. After this patch
> > > this operation will be done without holding mmap_lock, however per
> > > https://elixir.bootlin.com/linux/latest/source/include/linux/mm_types.h#L582
> > > this function has to be called with mmap_lock held for read. Indeed
> > > with debug flags enabled you should hit this assertion:
> > > https://elixir.bootlin.com/linux/latest/source/mm/madvise.c#L96.
>
> The documentation on the first link says to hold the lock or take a
> reference, but then we assert the lock.  If you take a reference to the
> anon vma name, then we will trigger the assert.  Either the
> documentation needs changing or the assert is incorrect - or I'm missing
> something?
>
> >
> > Sigh... Ok, what's the suggestion then? Should it be some variant of
> > mmap_assert_locked() || vma_assert_locked() logic, or it's not so
> > simple?
> >
> > Maybe I should just drop the CONFIG_PER_VMA_LOCK changes for now until
> > all these gotchas are figured out for /proc/<pid>/maps anyway, and
> > then we can adapt both text-based and ioctl-based /proc/<pid>/maps
> > APIs on top of whatever the final approach will end up being the right
> > one?
> >
> > Liam, any objections to this? The whole point of this patch set is to
> > add a new API, not all the CONFIG_PER_VMA_LOCK gotchas. My
> > implementation is structured in a way that should be easily amenable
> > to CONFIG_PER_VMA_LOCK changes, but if there are a few more subtle
> > things that need to be figured for existing text-based
> > /proc/<pid>/maps anyways, I think it would be best to use mmap_lock
> > for now for this new API, and then adopt the same final
> > CONFIG_PER_VMA_LOCK-aware solution.
>
> The reason I was hoping to have the new interface use the per-vma
> locking from the start is to ensure the guarantees that we provide to
> the users would not change.  We'd also avoid shifting to yet another
> mmap_lock users.
>

Yep, it's completely understandable. And you see that I changed the
structure quite a lot to abstract away mmap_lock vs vm_lock details.
I'm afraid anon_vma_name() is quite an obstacle, unfortunately, and
seems like it should be addressed first, but I'm just not qualified
enough to do this.

> I also didn't think it would complicate your series too much, so I
> understand why you want to revert to the old locking semantics.  I'm
> fine with you continuing with the series on the old lock.  Thanks for
> trying to make this work.
>

I'm happy to keep the existing structure of the code, and
(intentionally) all the CONFIG_PER_VMA_LOCK logic is in separate
patches, so it's easy to do. I'd love to help adopt a per-VMA lock
once all the pieces are figured out. Hopefully anon_vma_name() is the
last one remaining :) So please keep me cc'ed on relevant patches.

As I mentioned, I just don't feel like I would be able to solve the
anon_vma_name() problem, but of course I wouldn't want to be
completely blocked by it as well.

> Regards,
> Liam