Re: [PATCH v3 7/8] mm/maps: read proc/pid/maps under RCU

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Andrii Nakryiko <andrii.nakryiko@gmail.com> [250423 18:06]:
> On Wed, Apr 23, 2025 at 2:49 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Tue, Apr 22, 2025 at 3:49 PM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Fri, Apr 18, 2025 at 10:50 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > > >
> > > > With maple_tree supporting vma tree traversal under RCU and vma and
> > > > its important members being RCU-safe, /proc/pid/maps can be read under
> > > > RCU and without the need to read-lock mmap_lock. However vma content
> > > > can change from under us, therefore we make a copy of the vma and we
> > > > pin pointer fields used when generating the output (currently only
> > > > vm_file and anon_name). Afterwards we check for concurrent address
> > > > space modifications, wait for them to end and retry. While we take
> > > > the mmap_lock for reading during such contention, we do that momentarily
> > > > only to record new mm_wr_seq counter. This change is designed to reduce
> > >
> > > This is probably a stupid question, but why do we need to take a lock
> > > just to record this counter? uprobes get away without taking mmap_lock
> > > even for reads, and still record this seq counter. And then detect
> > > whether there were any modifications in between. Why does this change
> > > need more heavy-weight mmap_read_lock to do speculative reads?
> >
> > Not a stupid question. mmap_read_lock() is used to wait for the writer
> > to finish what it's doing and then we continue by recording a new
> > sequence counter value and call mmap_read_unlock. This is what
> > get_vma_snapshot() does. But your question made me realize that we can
> > optimize m_start() further by not taking mmap_read_lock at all.
> > Instead of taking mmap_read_lock then doing drop_mmap_lock() we can
> > try mmap_lock_speculate_try_begin() and only if it fails do the same
> > dance we do in the get_vma_snapshot(). I think that should work.
> 
> Ok, yeah, it would be great to avoid taking a lock in a common case!

We can check this counter once per 4k block and maintain the same
'tearing' that exists today instead of per-vma.  Not that anyone said
they had an issue with changing it, but since we're on this road anyways
I'd thought I'd point out where we could end up.

I am concerned about live locking in either scenario, but I haven't
looked too deep into this pattern.

I also don't love (as usual) the lack of ensured forward progress.

It seems like we have four cases for the vm area state now:
1. we want to read a stable vma or set of vmas (per-vma locking)
2. we want to read a stable mm state for reading (the very short named
mmap_lock_speculate_try_begin)
3. we ensure a stable vma/mm state for reading (mmap read lock)
4. we are writing - get out of my way (mmap write lock).

Cheers,
Liam