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

[Andrii Nakryiko]

On Wed, Oct 9, 2024 at 3:36 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Fri, Oct 04, 2024 at 12:58:00PM GMT, Andrii Nakryiko wrote:
> > On Fri, Oct 4, 2024 at 1:01 AM Christian Brauner <brauner@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > > > > Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> > > > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > > > ---
> > > >
> > > > Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>
> > >
> > > 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.
>
> Please drop this patch and pull the following tag which adds
> SLAB_TYPE_SAFE_BY_RCU protection for FMODE_BACKING files aligning them
> with regular files lifetime (even though not needed). The branch the tag
> is based on is stable and won't change anymore:
>
> git pull -S git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git tags/vfs-6.13.for-bpf.file

Ok, will drop. It will on Peter to pull this tag into tip/perf/core,
but I'll mention all this in the cover letter (and will pull locally
for testing, of course). Thanks.