linux-kernel - Re: [RFC PATCH] Page Cache Allowing Hard Interrupts

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Date:   Tue, 10 Aug 2021 13:09:45 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Vishal Moola <vishal.moola@...il.com>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] Page Cache Allowing Hard Interrupts

On Tue, Aug 10, 2021 at 09:15:28AM +0100, Christoph Hellwig wrote:
> Stupid question, but where do we ever do page cache interaction from
> soft irq context?

test_clear_page_writeback() happens in _some_ interrupt context (ie
the io completion path).  We had been under the impression that it was
always actually softirq context, and so this patch was safe.  However,
it's now clear that some drivers are calling it from hardirq context.
Writeback completions are clearly not latency sensitive and so can
be delayed from hardirq to softirq context without any problem, so I
think fixing this is just going to be a matter of tagging requests as
"complete in softirq context" and ensuring that blk_mq_raise_softirq()
is called for them.

Assuming that DIO write completions _are_ latency-sensitive, of course.
Maybe all write completions could be run in softirqs.