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

[Christian Brauner]

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.