Re: [PATCH v4 tip/perf/core 0/4] uprobes,mm: speculative lockless VMA-to-uprobe lookup

Andrii Nakryiko
2024-11-11T17:26:29Z

On Tue, Nov 5, 2024 at 6:01 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sun, Oct 27, 2024 at 6:09 PM Andrii Nakryiko <andrii@kernel.org> wrote:
> >
> > Implement speculative (lockless) resolution of VMA to inode to uprobe,
> > bypassing the need to take mmap_lock for reads, if possible. First two patches
> > by Suren adds mm_struct helpers that help detect whether mm_struct was
> > changed, which is used by uprobe logic to validate that speculative results
> > can be trusted after all the lookup logic results in a valid uprobe instance.
> >
> > Patch #3 is a simplification to uprobe VMA flag checking, suggested by Oleg.
> >
> > And, finally, patch #4 is the speculative VMA-to-uprobe resolution logic
> > itself, and is the focal point of this patch set. It makes entry uprobes in
> > common case scale very well with number of CPUs, as we avoid any locking or
> > cache line bouncing between CPUs. See corresponding patch for details and
> > benchmarking results.
> >
> > Note, this patch set assumes that FMODE_BACKING files were switched to have
> > SLAB_TYPE_SAFE_BY_RCU semantics, which was recently done by Christian Brauner
> > in [0]. This change can be pulled into perf/core through stable
> > tags/vfs-6.13.for-bpf.file tag from [1].
> >
> > [0] https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?h=vfs-6.13.for-bpf.file&id=8b1bc2590af61129b82a189e9dc7c2804c34400e
> > [1] git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
> >
> > v3->v4:
> > - rebased and dropped data_race(), given mm_struct uses real seqcount (Peter);
> > v2->v3:
> > - dropped kfree_rcu() patch (Christian);
> > - added data_race() annotations for fields of vma and vma->vm_file which could
> > be modified during speculative lookup (Oleg);
> > - fixed int->long problem in stubs for mmap_lock_speculation_{start,end}(),
> > caught by Kernel test robot;
> > v1->v2:
> > - adjusted vma_end_write_all() comment to point out it should never be called
> > manually now, but I wasn't sure how ACQUIRE/RELEASE comments should be
> > reworded (previously requested by Jann), so I'd appreciate some help there
> > (Jann);
> > - int -> long change for mm_lock_seq, as agreed at LPC2024 (Jann, Suren, Liam);
> > - kfree_rcu_mightsleep() for FMODE_BACKING (Suren, Christian);
> > - vm_flags simplification in find_active_uprobe_rcu() and
> > find_active_uprobe_speculative() (Oleg);
> > - guard(rcu)() simplified find_active_uprobe_speculative() implementation.
> >
> > Andrii Nakryiko (2):
> > uprobes: simplify find_active_uprobe_rcu() VMA checks
> > uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution
> >
> > Suren Baghdasaryan (2):
> > mm: Convert mm_lock_seq to a proper seqcount
> > mm: Introduce mmap_lock_speculation_{begin|end}
> >
> > include/linux/mm.h | 12 ++---
> > include/linux/mm_types.h | 7 ++-
> > include/linux/mmap_lock.h | 87 ++++++++++++++++++++++++--------
> > kernel/events/uprobes.c | 47 ++++++++++++++++-
> > kernel/fork.c | 5 +-
> > mm/init-mm.c | 2 +-
> > tools/testing/vma/vma.c | 4 +-
> > tools/testing/vma/vma_internal.h | 4 +-
> > 8 files changed, 129 insertions(+), 39 deletions(-)
> >
> > --
> > 2.43.5
> >
>
> Hi!
>
> What's the status of this patch set? Are there any blockers for it to
> be applied to perf/core? MM folks are OK with landing the first two
> patches in perf/core, so hopefully we should be good to go?

Another week, another ping. Peter, what can I do to make this land? MM
parts are clearly ok with Andrew Morton, uprobe-side logic didn't
change (modulo inconsequential data_race() back and forth) since at
least August, was approved by Oleg, and seems to be very stable in
testing. I think it's time to let me forget about this patch set and
make actual use of it in production, please.