linux-kernel - Re: [RFC PATCH] vfs: Fix might sleep in load_unaligned

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Message-ID: <20251126181031.GA3538@ZenIV>
Date: Wed, 26 Nov 2025 18:10:31 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Xie Yuanbin <xieyuanbin1@...wei.com>
Cc: brauner@...nel.org, jack@...e.cz, linux@...linux.org.uk,
	will@...nel.org, nico@...xnic.net, akpm@...ux-foundation.org,
	hch@....de, jack@...e.com, wozizhi@...weicloud.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	lilinjie8@...wei.com, liaohua4@...wei.com,
	wangkefeng.wang@...wei.com, pangliyuan1@...wei.com
Subject: Re: [RFC PATCH] vfs: Fix might sleep in load_unaligned_zeropad()
 with rcu read lock held

On Wed, Nov 26, 2025 at 06:19:52PM +0800, Xie Yuanbin wrote:
> When the path is initialized with LOOKUP_RCU flag in path_init(), the
> rcu read lock will be acquired. Inside the rcu critical section,
> load_unaligned_zeropad() may be called. According to the comments of
> load_unaligned_zeropad(), when loading the memory, a page fault may be
> triggered in the very unlikely case.

> Add pagefault_disable() to handle this situation.

Way too costly, IMO.  That needs to be dealt with in page fault handler
and IIRC arm used to do that; did that get broken at some point?