Message-ID: <CAEf4BzYyj78ALj5X42u3UE4GiyBOpwWDoqy=w6sSLB0YRoHajw@mail.gmail.com>
Date: Tue, 6 Aug 2024 10:40:15 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, rostedt@...dmis.org, 
	Matthew Wilcox <willy@...radead.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, mingo@...nel.org, andrii@...nel.org, 
	linux-kernel@...r.kernel.org, oleg@...hat.com, jolsa@...nel.org, clm@...a.com, 
	bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH 00/10] perf/uprobe: Optimize uprobes

On Tue, Aug 6, 2024 at 7:51 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Mon, Aug 5, 2024 at 9:08 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Sun, Aug 4, 2024 at 4:22 PM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Sat, Aug 3, 2024 at 1:53 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > >
> > > > On Fri, Aug 02, 2024 at 10:47:15PM -0700, Andrii Nakryiko wrote:
> > > >
> > > > > Is there any reason why the approach below won't work?
> > > >
> > > > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > > > > index 8be9e34e786a..e21b68a39f13 100644
> > > > > --- a/kernel/events/uprobes.c
> > > > > +++ b/kernel/events/uprobes.c
> > > > > @@ -2251,6 +2251,52 @@ static struct uprobe
> > > > > *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swb
> > > > >         struct uprobe *uprobe = NULL;
> > > > >         struct vm_area_struct *vma;
> > > > >
> > > > > +#ifdef CONFIG_PER_VMA_LOCK
> > > > > +       vm_flags_t flags = VM_HUGETLB | VM_MAYEXEC | VM_MAYSHARE, vm_flags;
> > > > > +       struct file *vm_file;
> > > > > +       struct inode *vm_inode;
> > > > > +       unsigned long vm_pgoff, vm_start, vm_end;
> > > > > +       int vm_lock_seq;
> > > > > +       loff_t offset;
> > > > > +
> > > > > +       rcu_read_lock();
> > > > > +
> > > > > +       vma = vma_lookup(mm, bp_vaddr);
> > > > > +       if (!vma)
> > > > > +               goto retry_with_lock;
> > > > > +
> > > > > +       vm_lock_seq = READ_ONCE(vma->vm_lock_seq);
> > > >
> > > > So vma->vm_lock_seq is only updated on vma_start_write()
> > >
> > > yep, I've looked a bit more at the implementation now
> > >
> > > >
> > > > > +
> > > > > +       vm_file = READ_ONCE(vma->vm_file);
> > > > > +       vm_flags = READ_ONCE(vma->vm_flags);
> > > > > +       if (!vm_file || (vm_flags & flags) != VM_MAYEXEC)
> > > > > +               goto retry_with_lock;
> > > > > +
> > > > > +       vm_inode = READ_ONCE(vm_file->f_inode);
> > > > > +       vm_pgoff = READ_ONCE(vma->vm_pgoff);
> > > > > +       vm_start = READ_ONCE(vma->vm_start);
> > > > > +       vm_end = READ_ONCE(vma->vm_end);
> > > >
> > > > None of those are written with WRITE_ONCE(), so this buys you nothing.
> > > > Compiler could be updating them one byte at a time while you load some
> > > > franken-update.
> > > >
> > > > Also, if you're in the middle of split_vma() you might not get a
> > > > consistent set.
> > >
> > > I used READ_ONCE() only to prevent the compiler from re-reading those
> > > values. We assume those values are garbage anyways and double-check
> > > everything, so lack of WRITE_ONCE doesn't matter. Same for
> > > inconsistency if we are in the middle of split_vma().
> > >
> > > We use the result of all this speculative calculation only if we find
> > > a valid uprobe (which could be a false positive) *and* if we detect
> > > that nothing about VMA changed (which is what I got wrong, but
> > > honestly I was actually betting on others to help me get this right
> > > anyways).
> > >
> > > >
> > > > > +       if (bp_vaddr < vm_start || bp_vaddr >= vm_end)
> > > > > +               goto retry_with_lock;
> > > > > +
> > > > > +       offset = (loff_t)(vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vm_start);
> > > > > +       uprobe = find_uprobe_rcu(vm_inode, offset);
> > > > > +       if (!uprobe)
> > > > > +               goto retry_with_lock;
> > > > > +
> > > > > +       /* now double check that nothing about VMA changed */
> > > > > +       if (vm_lock_seq != READ_ONCE(vma->vm_lock_seq))
> > > > > +               goto retry_with_lock;
> > > >
> > > > Since vma->vma_lock_seq is only ever updated at vma_start_write() you're
> > > > checking you're in or after the same modification cycle.
> > > >
> > > > The point of sequence locks is to check you *IN* a modification cycle
> > > > and retry if you are. You're now explicitly continuing if you're in a
> > > > modification.
> > > >
> > > > You really need:
> > > >
> > > >    seq++;
> > > >    wmb();
> > > >
> > > >    ... do modification
> > > >
> > > >    wmb();
> > > >    seq++;
> > > >
> > > > vs
> > > >
> > > >   do {
> > > >           s = READ_ONCE(seq) & ~1;
> > > >           rmb();
> > > >
> > > >           ... read stuff
> > > >
> > > >   } while (rmb(), seq != s);
> > > >
> > > >
> > > > The thing to note is that seq will be odd while inside a modification
> > > > and even outside, further if the pre and post seq are both even but not
> > > > identical, you've crossed a modification and also need to retry.
> > > >
> > >
> > > Ok, I don't think I got everything you have written above, sorry. But
> > > let me explain what I think I need to do and please correct what I
> > > (still) got wrong.
> > >
> > > a) before starting speculation,
> > >   a.1) read and remember current->mm->mm_lock_seq (using
> > > smp_load_acquire(), right?)
> > >   a.2) read vma->vm_lock_seq (using smp_load_acquire() I presume)
> > >   a.3) if vm_lock_seq is odd, we are already modifying VMA, so bail
> > > out, try with proper mmap_lock
> > > b) proceed with the inode pointer fetch and offset calculation as I've coded it
> > > c) lookup uprobe by inode+offset, if failed -- bail out (if succeeded,
> > > this could still be wrong)
> > > d) re-read vma->vm_lock_seq, if it changed, we started modifying/have
> > > already modified VMA, bail out
> > > e) re-read mm->mm_lock_seq, if that changed -- presume VMA got
> > > modified, bail out
> > >
> > > At this point we should have a guarantee that nothing about mm
> > > changed, nor that VMA started being modified during our speculative
> > > calculation+uprobe lookup. So if we found a valid uprobe, it must be a
> > > correct one that we need.
> > >
> > > Is that enough? Any holes in the approach? And thanks for thoroughly
> > > thinking about this, btw!
> >
> > Ok, with slight modifications to the details of the above (e.g., there
> > is actually no "odd means VMA is being modified" thing with
> > vm_lock_seq), I ended up with the implementation below. Basically we
> > validate that mm->mm_lock_seq didn't change and that vm_lock_seq !=
> > mm_lock_seq (which otherwise would mean "VMA is being modified").
> > There is a possibility that vm_lock_seq == mm_lock_seq just by
> > accident, which is not a correctness problem, we'll just fallback to
> > locked implementation until something about VMA or mm_struct itself
> > changes. Which is fine, and if mm folks ever change this locking
> > schema, this might go away.
> >
> > If this seems on the right track, I think we can just move
> > mm_start_vma_specuation()/mm_end_vma_speculation() into
> > include/linux/mm.h.
> >
> > And after thinking a bit more about READ_ONCE() usage, I changed them
> > to data_race() to not trigger KCSAN warnings. Initially I kept
> > READ_ONCE() only around vma->vm_file access, but given we never change
> > it until vma is freed and reused (which would be prevented by
> > guard(rcu)), I dropped READ_ONCE() and only added data_race(). And
> > even data_race() is probably not necessary.
> >
> > Anyways, please see the patch below. Would be nice if mm folks
> > (Suren?) could confirm that this is not broken.
>
> Hi Andrii,
> Sorry, I'm catching up on my emails and will get back to this to read
> through the discussion later today.

Great, thank you! I appreciate you taking a thorough look!

> One obvious thing that is problematic in this whole schema is the fact
> that vm_file is not RCU-safe as I mentioned before. See below.
>
> >
> >
> >
> > Author: Andrii Nakryiko <andrii@...nel.org>
> > Date:   Fri Aug 2 22:16:40 2024 -0700
> >
> >     uprobes: add speculative lockless VMA to inode resolution
> >
> >     Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> >
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 3de311c56d47..bee7a929ff02 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -2244,6 +2244,70 @@ static int is_trap_at_addr(struct mm_struct
> > *mm, unsigned long vaddr)
> >         return is_trap_insn(&opcode);
> >  }
> >
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +static inline void mm_start_vma_speculation(struct mm_struct *mm, int
> > *mm_lock_seq)
> > +{
> > +       *mm_lock_seq = smp_load_acquire(&mm->mm_lock_seq);
> > +}
> > +
> > +/* returns true if speculation was safe (no mm and vma modification
> > happened) */
> > +static inline bool mm_end_vma_speculation(struct vm_area_struct *vma,
> > int mm_lock_seq)
> > +{
> > +       int mm_seq, vma_seq;
> > +
> > +       mm_seq = smp_load_acquire(&vma->vm_mm->mm_lock_seq);
> > +       vma_seq = READ_ONCE(vma->vm_lock_seq);
> > +
> > +       return mm_seq == mm_lock_seq && vma_seq != mm_seq;
> > +}
> > +
> > +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 mm_lock_seq;
> > +       loff_t offset;
> > +
> > +       guard(rcu)();
> > +
> > +       mm_start_vma_speculation(mm, &mm_lock_seq);
> > +
> > +       vma = vma_lookup(mm, bp_vaddr);
> > +       if (!vma)
> > +               return NULL;
> > +
> > +       vm_file = data_race(vma->vm_file);
>
> Consider what happens if remove_vma() is racing with this code and at
> this point it calls fput(vma->vm_file). Your vma will be valid (it's
> freed after RCU grace period) but vma->vm_file can be freed from under
> you. For this to work vma->vm_file should be made RCU-safe.

Note that I'm adding SLAB_TYPESAFE_BY_RCU to files_cachep, as
suggested by Matthew. I thought that would be enough to ensure that
vm_file's memory won't be freed until after the RCU grace period. It's
ok if file struct itself is reused for another file (we should detect
that as vma/mm change with sequence numbers).

> Thanks,
> Suren.
>
> > +       if (!vm_file || (vma->vm_flags & flags) != VM_MAYEXEC)
> > +               return NULL;
> > +
> > +       vm_inode = data_race(vm_file->f_inode);
> > +       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)
> > +               return NULL;
> > +
> > +       /* now double check that nothing about MM and VMA changed */
> > +       if (!mm_end_vma_speculation(vma, mm_lock_seq))
> > +               return NULL;
> > +
> > +       /* happy case, we speculated successfully */
> > +       return uprobe;
> > +}
> > +#else /* !CONFIG_PER_VMA_LOCK */
> > +static struct uprobe *find_active_uprobe_speculative(unsigned long bp_vaddr)
> > +{
> > +       return NULL;
> > +}
> > +#endif /* CONFIG_PER_VMA_LOCK */
> > +
> >  /* assumes being inside RCU protected region */
> >  static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr,
> > int *is_swbp)
> >  {
> > @@ -2251,6 +2315,10 @@ static struct uprobe
> > *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swb
> >         struct uprobe *uprobe = NULL;
> >         struct vm_area_struct *vma;
> >
> > +       uprobe = find_active_uprobe_speculative(bp_vaddr);
> > +       if (uprobe)
> > +               return uprobe;
> > +
> >         mmap_read_lock(mm);
> >         vma = vma_lookup(mm, bp_vaddr);
> >         if (vma) {
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index cc760491f201..211a84ee92b4 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -3160,7 +3160,7 @@ void __init proc_caches_init(void)
> >                         NULL);
> >         files_cachep = kmem_cache_create("files_cache",
> >                         sizeof(struct files_struct), 0,
> > -                       SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT,
> > +
> > SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT|SLAB_TYPESAFE_BY_RCU,
> >                         NULL);
> >         fs_cachep = kmem_cache_create("fs_cache",
> >                         sizeof(struct fs_struct), 0,
> >
> >
> > >
> > > P.S. This is basically the last big blocker towards linear uprobes
> > > scalability with the number of active CPUs. I have
> > > uretprobe+SRCU+timeout implemented and it seems to work fine, will
> > > post soon-ish.
> > >
> > > P.P.S Also, funny enough, below was another big scalability limiter
> > > (and the last one) :) I'm not sure if we can just drop it, or I should
> > > use per-CPU counter, but with the below change and speculative VMA
> > > lookup (however buggy, works ok for benchmarking), I finally get
> > > linear scaling of uprobe triggering throughput with number of CPUs. We
> > > are very close.
> > >
> > > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> > > index f7443e996b1b..64c2bc316a08 100644
> > > --- a/kernel/trace/trace_uprobe.c
> > > +++ b/kernel/trace/trace_uprobe.c
> > > @@ -1508,7 +1508,7 @@ static int uprobe_dispatcher(struct
> > > uprobe_consumer *con, struct pt_regs *regs)
> > >         int ret = 0;
> > >
> > >         tu = container_of(con, struct trace_uprobe, consumer);
> > > -       tu->nhit++;
> > > +       //tu->nhit++;
> > >
> > >         udd.tu = tu;
> > >         udd.bp_addr = instruction_pointer(regs);
> > >
> > >
> > > > > +
> > > > > +       /* happy case, we speculated successfully */
> > > > > +       rcu_read_unlock();
> > > > > +       return uprobe;
> > > > > +
> > > > > +retry_with_lock:
> > > > > +       rcu_read_unlock();
> > > > > +       uprobe = NULL;
> > > > > +#endif
> > > > > +
> > > > >         mmap_read_lock(mm);
> > > > >         vma = vma_lookup(mm, bp_vaddr);
> > > > >         if (vma) {
> > > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > > index cc760491f201..211a84ee92b4 100644
> > > > > --- a/kernel/fork.c
> > > > > +++ b/kernel/fork.c
> > > > > @@ -3160,7 +3160,7 @@ void __init proc_caches_init(void)
> > > > >                         NULL);
> > > > >         files_cachep = kmem_cache_create("files_cache",
> > > > >                         sizeof(struct files_struct), 0,
> > > > > -                       SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT,
> > > > > + SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT|SLAB_TYPESAFE_BY_RCU,
> > > > >                         NULL);
> > > > >         fs_cachep = kmem_cache_create("fs_cache",
> > > > >                         sizeof(struct fs_struct), 0,

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