Re: [PATCH v3 8/8] mm/maps: execute PROCMAP_QUERY ioctl under RCU

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Apr 18, 2025 at 10:50 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> Utilize speculative vma lookup to find and snapshot a vma without
> taking mmap_lock during PROCMAP_QUERY ioctl execution. Concurrent
> address space modifications are detected and the lookup is retried.
> While we take the mmap_lock for reading during such contention, we
> do that momentarily only to record new mm_wr_seq counter.

PROCMAP_QUERY is an even more obvious candidate for fully lockless
speculation, IMO (because it's more obvious that vma's use is
localized to do_procmap_query(), instead of being spread across
m_start/m_next and m_show as with seq_file approach). We do
rcu_read_lock(), mmap_lock_speculate_try_begin(), query for VMA (no
mmap_read_lock), use that VMA to produce (speculative) output, and
then validate that VMA or mm_struct didn't change with
mmap_lock_speculate_retry(). If it did - retry, if not - we are done.
No need for vma_copy and any gets/puts, no?

> This change is designed to reduce mmap_lock contention and prevent
> PROCMAP_QUERY ioctl calls (often a low priority task, such as
> monitoring/data collection services) from blocking address space
> updates.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  fs/proc/task_mmu.c | 63 ++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 55 insertions(+), 8 deletions(-)
>

[...]