Re: [PATCH RFC v3 13/13] uprobes: add speculative lockless VMA to inode resolution

Suren Baghdasaryan
2024-08-15T19:44:54Z

On Thu, Aug 15, 2024 at 11:58 AM Jann Horn <jannh@google.com> wrote:
>
> +brauner for "struct file" lifetime
>
> On Thu, Aug 15, 2024 at 7:45 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > On Thu, Aug 15, 2024 at 9:47 AM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Thu, Aug 15, 2024 at 6:44 AM Mateusz Guzik <mjguzik@gmail.com> wrote:
> > > >
> > > > On Tue, Aug 13, 2024 at 08:36:03AM -0700, Suren Baghdasaryan wrote:
> > > > > On Mon, Aug 12, 2024 at 11:18 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
> > > > > >
> > > > > > On Mon, Aug 12, 2024 at 09:29:17PM -0700, Andrii Nakryiko wrote:
> > > > > > > Now that files_cachep is SLAB_TYPESAFE_BY_RCU, we can safely access
> > > > > > > vma->vm_file->f_inode lockless only under rcu_read_lock() protection,
> > > > > > > attempting uprobe look up speculatively.
>
> Stupid question: Is this uprobe stuff actually such a hot codepath
> that it makes sense to optimize it to be faster than the page fault
> path?
>
> (Sidenote: I find it kinda interesting that this is sort of going back
> in the direction of the old Speculative Page Faults design.)
>
> > > > > > > We rely on newly added mmap_lock_speculation_{start,end}() helpers to
> > > > > > > validate that mm_struct stays intact for entire duration of this
> > > > > > > speculation. If not, we fall back to mmap_lock-protected lookup.
> > > > > > >
> > > > > > > This allows to avoid contention on mmap_lock in absolutely majority of
> > > > > > > cases, nicely improving uprobe/uretprobe scalability.
> > > > > > >
> > > > > >
> > > > > > Here I have to admit to being mostly ignorant about the mm, so bear with
> > > > > > me. :>
> > > > > >
> > > > > > I note the result of find_active_uprobe_speculative is immediately stale
> > > > > > in face of modifications.
> > > > > >
> > > > > > The thing I'm after is that the mmap_lock_speculation business adds
> > > > > > overhead on archs where a release fence is not a de facto nop and I
> > > > > > don't believe the commit message justifies it. Definitely a bummer to
> > > > > > add merely it for uprobes. If there are bigger plans concerning it
> > > > > > that's a different story of course.
> > > > > >
> > > > > > With this in mind I have to ask if instead you could perhaps get away
> > > > > > with the already present per-vma sequence counter?
> > > > >
> > > > > per-vma sequence counter does not implement acquire/release logic, it
> > > > > relies on vma->vm_lock for synchronization. So if we want to use it,
> > > > > we would have to add additional memory barriers here. This is likely
> > > > > possible but as I mentioned before we would need to ensure the
> > > > > pagefault path does not regress. OTOH mm->mm_lock_seq already halfway
> > > > > there (it implements acquire/release logic), we just had to ensure
> > > > > mmap_write_lock() increments mm->mm_lock_seq.
> > > > >
> > > > > So, from the release fence overhead POV I think whether we use
> > > > > mm->mm_lock_seq or vma->vm_lock, we would still need a proper fence
> > > > > here.
> > > > >
> > > >
> > > > Per my previous e-mail I'm not particularly familiar with mm internals,
> > > > so I'm going to handwave a little bit with my $0,03 concerning multicore
> > > > in general and if you disagree with it that's your business. For the
> > > > time being I have no interest in digging into any of this.
> > > >
> > > > Before I do, to prevent this thread from being a total waste, here are
> > > > some remarks concerning the patch with the assumption that the core idea
> > > > lands.
> > > >
> > > > From the commit message:
> > > > > Now that files_cachep is SLAB_TYPESAFE_BY_RCU, we can safely access
> > > > > vma->vm_file->f_inode lockless only under rcu_read_lock() protection,
> > > > > attempting uprobe look up speculatively.
> > > >
> > > > Just in case I'll note a nit that this paragraph will need to be removed
> > > > since the patch adding the flag is getting dropped.
> > >
> > > Yep, of course, I'll update all that for the next revision (I'll wait
> > > for non-RFC patches to land first before reposting).
> > >
> > > >
> > > > A non-nit which may or may not end up mattering is that the flag (which
> > > > *is* set on the filep slab cache) makes things more difficult to
> > > > validate. Normal RCU usage guarantees that the object itself wont be
> > > > freed as long you follow the rules. However, the SLAB_TYPESAFE_BY_RCU
> > > > flag weakens it significantly -- the thing at hand will always be a
> > > > 'struct file', but it may get reallocated to *another* file from under
> > > > you. Whether this aspect plays a role here I don't know.
> > >
> > > Yes, that's ok and is accounted for. We care about that memory not
> > > going even from under us (I'm not even sure if it matters that it is
> > > still a struct file, tbh; I think that shouldn't matter as we are
> > > prepared to deal with completely garbage values read from struct
> > > file).
> >
> > Correct, with SLAB_TYPESAFE_BY_RCU we do need an additional check that
> > vma->vm_file has not been freed and reused. That's where
> > mmap_lock_speculation_{start|end} helps us. For vma->vm_file to change
> > from under us one would have to take mmap_lock for write. If that
> > happens mmap_lock_speculation_{start|end} should detect that and
> > terminate our speculation.
> >
> > >
> > > >
> > > > > +static struct uprobe *find_active_uprobe_speculative(unsigned long bp_vaddr)
> > > > > +{
> > > > > + const vm_flags_t flags = VM_HUGETLB | VM_MAYEXEC | VM_MAYSHARE;
> > > > > + struct mm_struct *mm = current->mm;
> > > > > + struct uprobe *uprobe;
> > > > > + struct vm_area_struct *vma;
> > > > > + struct file *vm_file;
> > > > > + struct inode *vm_inode;
> > > > > + unsigned long vm_pgoff, vm_start;
> > > > > + int seq;
> > > > > + loff_t offset;
> > > > > +
> > > > > + if (!mmap_lock_speculation_start(mm, &seq))
> > > > > + return NULL;
> > > > > +
> > > > > + rcu_read_lock();
> > > > > +
> > > >
> > > > I don't think there is a correctness problem here, but entering rcu
> > > > *after* deciding to speculatively do the lookup feels backwards.
> > >
> > > RCU should protect VMA and file, mm itself won't go anywhere, so this seems ok.
> > >
> > > >
> > > > > + vma = vma_lookup(mm, bp_vaddr);
> > > > > + if (!vma)
> > > > > + goto bail;
> > > > > +
> > > > > + vm_file = data_race(vma->vm_file);
> > > > > + if (!vm_file || (vma->vm_flags & flags) != VM_MAYEXEC)
> > > > > + goto bail;
> > > > > +
> > > >
> > > > If vma teardown is allowed to progress and the file got fput'ed...
> > > >
> > > > > + vm_inode = data_race(vm_file->f_inode);
> > > >
> > > > ... the inode can be NULL, I don't know if that's handled.
> > > >
> > >
> > > Yep, inode pointer value is part of RB-tree key, so if it's NULL, we
> > > just won't find a matching uprobe. Same for any other "garbage"
> > > f_inode value. Importantly, we never should dereference such inode
> > > pointers, at least until we find a valid uprobe (in which case we keep
> > > inode reference to it).
> > >
> > > > More importantly though, per my previous description of
> > > > SLAB_TYPESAFE_BY_RCU, by now the file could have been reallocated and
> > > > the inode you did find is completely unrelated.
> > > >
> > > > I understand the intent is to backpedal from everything should the mm
> > > > seqc change, but the above may happen to matter.
> > >
> > > Yes, I think we took that into account. All that we care about is
> > > memory "type safety", i.e., even if struct file's memory is reused,
> > > it's ok, we'll eventually detect the change and will discard wrong
> > > uprobe that we might by accident lookup (though probably in most cases
> > > we just won't find a uprobe at all).
> > >
> > > >
> > > > > + vm_pgoff = data_race(vma->vm_pgoff);
> > > > > + vm_start = data_race(vma->vm_start);
> > > > > +
> > > > > + offset = (loff_t)(vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vm_start);
> > > > > + uprobe = find_uprobe_rcu(vm_inode, offset);
> > > > > + if (!uprobe)
> > > > > + goto bail;
> > > > > +
> > > > > + /* now double check that nothing about MM changed */
> > > > > + if (!mmap_lock_speculation_end(mm, seq))
> > > > > + goto bail;
> > > >
> > > > This leaks the reference obtained by find_uprobe_rcu().
> > >
> > > find_uprobe_rcu() doesn't obtain a reference, uprobe is RCU-protected,
> > > and if caller need a refcount bump it will have to use
> > > try_get_uprobe() (which might fail).
> > >
> > > >
> > > > > +
> > > > > + rcu_read_unlock();
> > > > > +
> > > > > + /* happy case, we speculated successfully */
> > > > > + return uprobe;
> > > > > +bail:
> > > > > + rcu_read_unlock();
> > > > > + return NULL;
> > > > > +}
> > > >
> > > > Now to some handwaving, here it is:
> > > >
> > > > The core of my concern is that adding more work to down_write on the
> > > > mmap semaphore comes with certain side-effects and plausibly more than a
> > > > sufficient speed up can be achieved without doing it.
> >
> > AFAIK writers of mmap_lock are not considered a fast path. In a sense
> > yes, we made any writer a bit heavier but OTOH we also made
> > mm->mm_lock_seq a proper sequence count which allows us to locklessly
> > check if mmap_lock is write-locked. I think you asked whether there
> > will be other uses for mmap_lock_speculation_{start|end} and yes. For
> > example, I am planning to use them for printing /proc/{pid}/maps
> > without taking mmap_lock (when it's uncontended).
>
> What would be the goal of this - to avoid cacheline bouncing of the
> mmap lock between readers? Or to allow mmap_write_lock() to preempt
> /proc/{pid}/maps readers who started out uncontended?

The latter, from my early patchset which I need to refine
(https://lore.kernel.org/all/20240123231014.3801041-3-surenb@google.com/):

This change is designed to reduce mmap_lock contention and prevent a
process reading /proc/pid/maps files (often a low priority task, such as
monitoring/data collection services) from blocking address space updates.

>
> Is the idea that you'd change show_map_vma() to first do something
> like get_file_active() to increment the file refcount (because
> otherwise the dentry can be freed under you and you need the dentry
> for path printing), then recheck your sequence count on the mm or vma
> (to avoid accessing the dentry of an unrelated file that hasn't become
> userspace-visible yet and may not have a proper dentry pointer yet),
> then print the file path, drop the file reference again, and in the
> end recheck the sequence count again before actually returning the
> printed data to userspace?

Yeah, you can see the details in that link I posted above. See
get_vma_snapshot() function.

>
> > If we have VMA seq
> > counter-based detection it would be better (see below).
> >
> > > >
> > > > An mm-wide mechanism is just incredibly coarse-grained and it may happen
> > > > to perform poorly when faced with a program which likes to mess with its
> > > > address space -- the fast path is going to keep failing and only
> > > > inducing *more* overhead as the code decides to down_read the mmap
> > > > semaphore.
> > > >
> > > > Furthermore there may be work currently synchronized with down_write
> > > > which perhaps can transition to "merely" down_read, but by the time it
> > > > happens this and possibly other consumers expect a change in the
> > > > sequence counter, messing with it.
> > > >
> > > > To my understanding the kernel supports parallel faults with per-vma
> > > > locking. I would find it surprising if the same machinery could not be
> > > > used to sort out uprobe handling above.
> >
> > From all the above, my understanding of your objection is that
> > checking mmap_lock during our speculation is too coarse-grained and
> > you would prefer to use the VMA seq counter to check that the VMA we
> > are working on is unchanged. I agree, that would be ideal. I had a
> > quick chat with Jann about this and the conclusion we came to is that
> > we would need to add an additional smp_wmb() barrier inside
> > vma_start_write() and a smp_rmb() in the speculation code:
> >
> > static inline void vma_start_write(struct vm_area_struct *vma)
> > {
> > int mm_lock_seq;
> >
> > if (__is_vma_write_locked(vma, &mm_lock_seq))
> > return;
> >
> > down_write(&vma->vm_lock->lock);
> > /*
> > * We should use WRITE_ONCE() here because we can have concurrent reads
> > * from the early lockless pessimistic check in vma_start_read().
> > * We don't really care about the correctness of that early check, but
> > * we should use WRITE_ONCE() for cleanliness and to keep KCSAN happy.
> > */
> > WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);
> > + smp_wmb();
> > up_write(&vma->vm_lock->lock);
> > }
> >
> > Note: up_write(&vma->vm_lock->lock) in the vma_start_write() is not
> > enough because it's one-way permeable (it's a "RELEASE operation") and
> > later vma->vm_file store (or any other VMA modification) can move
> > before our vma->vm_lock_seq store.
> >
> > This makes vma_start_write() heavier but again, it's write-locking, so
> > should not be considered a fast path.
> > With this change we can use the code suggested by Andrii in
> > https://lore.kernel.org/all/CAEf4BzZeLg0WsYw2M7KFy0+APrPaPVBY7FbawB9vjcA2+6k69Q@mail.gmail.com/
> > with an additional smp_rmb():
> >
> > rcu_read_lock()
> > vma = find_vma(...)
> > if (!vma) /* bail */
>
> And maybe add some comments like:
>
> /*
> * Load the current VMA lock sequence - we will detect if anyone concurrently
> * locks the VMA after this point.
> * Pairs with smp_wmb() in vma_start_write().
> */
> > vm_lock_seq = smp_load_acquire(&vma->vm_lock_seq);
> /*
> * Now we just have to detect if the VMA is already locked with its current
> * sequence count.
> *
> * The following load is ordered against the vm_lock_seq load above (using
> * smp_load_acquire() for the load above), and pairs with implicit memory
> * ordering between the mm_lock_seq write in mmap_write_unlock() and the
> * vm_lock_seq write in the next vma_start_write() after that (which can only
> * occur after an mmap_write_lock()).
> */
> > mm_lock_seq = smp_load_acquire(&vma->mm->mm_lock_seq);
> > /* I think vm_lock has to be acquired first to avoid the race */
> > if (mm_lock_seq == vm_lock_seq)
> > /* bail, vma is write-locked */
> > ... perform uprobe lookup logic based on vma->vm_file->f_inode ...
> /*
> * Order the speculative accesses above against the following vm_lock_seq
> * recheck.
> */
> > smp_rmb();
> > if (vma->vm_lock_seq != vm_lock_seq)
>
> (As I said on the other thread: Since this now relies on
> vma->vm_lock_seq not wrapping back to the same value for correctness,
> I'd like to see vma->vm_lock_seq being at least an "unsigned long", or
> even better, an atomic64_t... though I realize we don't currently do
> that for seqlocks either.)
>
> > /* bail, VMA might have changed */
> >
> > The smp_rmb() is needed so that vma->vm_lock_seq load does not get
> > reordered and moved up before speculation.
> >
> > I'm CC'ing Jann since he understands memory barriers way better than
> > me and will keep me honest.