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Date: Fri, 28 Aug 2026 18:10:35 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Wenjie Qi <qwjhust@gmail.com>
Cc: willy@infradead.org, jack@suse.cz, linux-fsdevel@vger.kernel.org,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org, baohua@kernel.org, Wenjie
 Qi <qiwenjie@xiaomi.com>
Subject: Re: [PATCH] mm: filemap: tighten dropbehind completion context
 check
Message-Id: <20260828181035.ce6e23970d37e38b64818f10@linux-foundation.org>
In-Reply-To: <20260820142956.1414337-1-qiwenjie@xiaomi.com>
References: <20260820142956.1414337-1-qiwenjie@xiaomi.com>
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On Thu, 20 Aug 2026 22:29:56 +0800 Wenjie Qi <qwjhust@gmail.com> 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.
> 
> ...
>
> +static bool folio_dropbehind_in_atomic(void)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPTION) && rcu_preempt_depth())
> +		return true;
> +	if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
> +		return true;
> +	return !preemptible();
> +}

Cripes.

There's nothing mm-specific about this function.  If we have a use-case
for such a thing then surely the function should be kernel-wide, it
should live at the sched/rcu/etc layer and it should be elaborately
documented?