Message-ID: <CAJuCfpELNoDrVyyNV+fuB7ju77pqyj0rD0gOkLVX+RHKTxXGCA@mail.gmail.com>
Date: Fri, 26 Jul 2024 18:29:44 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Peter Zijlstra <peterz@...radead.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, rostedt@...dmis.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 Fri, Jul 26, 2024 at 5:20 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Mon, Jul 22, 2024 at 12:09 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
> >
> > On Wed, Jul 10, 2024 at 2:40 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Wed, Jul 10, 2024 at 11:16:31AM +0200, Peter Zijlstra wrote:
> > >
> > > > If it were an actual sequence count, I could make it work, but sadly,
> > > > not. Also, vma_end_write() seems to be missing :-( If anything it could
> > > > be used to lockdep annotate the thing.
> >
> > Thanks Matthew for forwarding me this discussion!
> >
> > > >
> > > > Mooo.. I need to stare more at this to see if perhaps it can be made to
> > > > work, but so far, no joy :/
> > >
> > > See, this is what I want, except I can't close the race against VMA
> > > modification because of that crazy locking scheme :/
> >
> > Happy to explain more about this crazy locking scheme. The catch is
> > that we can write-lock a VMA only while holding mmap_lock for write
> > and we unlock all write-locked VMAs together when we drop that
> > mmap_lock:
> >
> > mmap_write_lock(mm);
> > vma_start_write(vma1);
> > vma_start_write(vma2);
> > ...
> > mmap_write_unlock(mm); -> vma_end_write_all(mm); // unlocks all locked vmas
> >
> > This is done because oftentimes we need to lock multiple VMAs when
> > modifying the address space (vma merge/split) and unlocking them
> > individually would be more expensive than unlocking them in bulk by
> > incrementing mm->mm_lock_seq.
> >
> > >
> > >
> > > --- a/kernel/events/uprobes.c
> > > +++ b/kernel/events/uprobes.c
> > > @@ -2146,11 +2146,58 @@ static int is_trap_at_addr(struct mm_str
> > >         return is_trap_insn(&opcode);
> > >  }
> > >
> > > -static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
> > > +#ifndef CONFIG_PER_VMA_LOCK
> > > +static struct uprobe *__find_active_uprobe(unsigned long bp_vaddr)
> > > +{
> > > +       return NULL;
> > > +}
> > > +#else
> >
> > IIUC your code below, you want to get vma->vm_file without locking the
> > VMA. I think under RCU that would have been possible if vma->vm_file
> > were RCU-safe, which it's not (we had discussions with Paul and
> > Matthew about that in
> > https://lore.kernel.org/all/CAJuCfpHW2=Zu+CHXL+5fjWxGk=CVix=C66ra+DmXgn6r3+fsXg@mail.gmail.com/).
> > Otherwise you could store the value of vma->vm_lock_seq before
> > comparing it with mm->mm_lock_seq, then do get_file(vma->file) and
> > then compare your locally stored vm_lock_seq against vma->vm_lock_seq
> > to see if VMA got locked for modification after we got the file. So,
> > unless I miss some other race, I think the VMA locking sequence does
> > not preclude you from implementing __find_active_uprobe() but
> > accessing vma->vm_file would be unsafe without some kind of locking.
>
> Hey Suren!
>
> I've haven't yet dug properly into this, but from quick checking
> around I think for the hot path (where this all matters), we really
> only want to get vma's underlying inode. vm_file itself is just a
> means to that end. If there is some clever way to do
> vma->vm_file->f_inode under RCU and without mmap_read_lock, that would
> be good enough, I think.

Hi Andrii,
Sorry, I'm not aware of any other way to get the inode from vma. Maybe
Matthew with his FS background can find a way?

>
> >
> > > +static struct uprobe *__find_active_uprobe(unsigned long bp_vaddr)
> > >  {
> > >         struct mm_struct *mm = current->mm;
> > >         struct uprobe *uprobe = NULL;
> > >         struct vm_area_struct *vma;
> > > +       MA_STATE(mas, &mm->mm_mt, bp_vaddr, bp_vaddr);
> > > +
> > > +       guard(rcu)();
> > > +
> > > +again:
> > > +       vma = mas_walk(&mas);
> > > +       if (!vma)
> > > +               return NULL;
> > > +
> > > +       /* vma_write_start() -- in progress */
> > > +       if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq))
> > > +               return NULL;
> > > +
> > > +       /*
> > > +        * Completely broken, because of the crazy vma locking scheme you
> > > +        * cannot avoid the per-vma rwlock and doing so means you're racy
> > > +        * against modifications.
> > > +        *
> > > +        * A simple actual seqcount would'be been cheaper and more usefull.
> > > +        */
> > > +
> > > +       if (!valid_vma(vma, false))
> > > +               return NULL;
> > > +
> > > +       struct inode = file_inode(vma->vm_file);
> > > +       loff_t offset = vaddr_to_offset(vma, bp_vaddr);
> > > +
> > > +       // XXX: if (vma_seq_retry(...)) goto again;
> > > +
> > > +       return find_uprobe(inode, offset);
> > > +}
> > > +#endif
> > > +
> > > +static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
> > > +{
> > > +       struct uprobe *uprobe = __find_active_uprobe(bp_vaddr)
> > > +       struct mm_struct *mm = current->mm;
> > > +       struct vm_area_struct *vma;
> > > +
> > > +       if (uprobe)
> > > +               return uprobe;
> > >
> > >         mmap_read_lock(mm);
> > >         vma = vma_lookup(mm, bp_vaddr);
> > >

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