Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Matthew Wilcox]

On Thu, Aug 20, 2026 at 10:29:56PM +0800, Wenjie Qi wrote:
> folio_end_dropbehind() uses in_task() to keep folio invalidation out of
> interrupt context. Task context alone is not sufficient: preemption can
> still be disabled, or the task can be in a preemptible RCU read-side
> critical section, while filemap_end_dropbehind() may reach
> folio_unmap_invalidate() and sleep.
> 
> Use the established conservative three-part atomic-context test: reject
> preemptible RCU read-side sections, reject configurations without
> PREEMPT_COUNT, and otherwise require a preemptible context. Unsafe
> completions retain the existing best-effort behavior and skip invalidation.

Have you seen this happen in practice, or is this based on code
examination?