Re: [PATCH v2 tip/perf/core 3/5] fs: add back RCU-delayed freeing of FMODE_BACKING file

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

On Fri, Oct 4, 2024 at 1:01 AM Christian Brauner <brauner@kernel.org> wrote:
>
> On Thu, Oct 03, 2024 at 11:13:54AM GMT, Christian Brauner wrote:
> > On Tue, Oct 01, 2024 at 03:52:05PM GMT, Andrii Nakryiko wrote:
> > > 6cf41fcfe099 ("backing file: free directly") switched FMODE_BACKING
> > > files to direct freeing as back then there were no use cases requiring
> > > RCU protected access to such files.
> > >
> > > Now, with speculative lockless VMA-to-uprobe lookup logic, we do need to
> > > have a guarantee that struct file memory is not going to be freed from
> > > under us during speculative check. So add back RCU-delayed freeing
> > > logic.
> > >
> > > We use headless kfree_rcu_mightsleep() variant, as file_free() is only
> > > called for FMODE_BACKING files in might_sleep() context.
> > >
> > > Suggested-by: Suren Baghdasaryan <surenb@google.com>
> > > Cc: Christian Brauner <brauner@kernel.org>
> > > Cc: Amir Goldstein <amir73il@gmail.com>
> > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > > ---
> >
> > Reviewed-by: Christian Brauner <brauner@kernel.org>
>
> Fwiw, I have another patch series for files that I'm testing that will
> require me to switch FMODE_BACKING to a SLAB_TYPSAFE_BY_RCU cache. That
> shouldn't matter for your use-case though.

Correct, we assume SLAB_TYPESAFE_BY_RCU semantics for the common case
anyways. But hopefully my change won't cause major merge conflicts
with your patch set.