linux-kernel - Re: [PATCH AUTOSEL 5.0 011/262] tracing: kdb: Fix ftdump to not sleep

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Message-ID: <20190328101342.GD19456@atrey.karlin.mff.cuni.cz>
Date:   Thu, 28 Mar 2019 11:13:42 +0100
From:   Pavel Machek <pavel@....cz>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH AUTOSEL 5.0 011/262] tracing: kdb: Fix ftdump to not sleep

> From: Douglas Anderson <dianders@...omium.org>
> 
> [ Upstream commit 31b265b3baaf55f209229888b7ffea523ddab366 ]
> 
> As reported back in 2016-11 [1], the "ftdump" kdb command triggers a
> BUG for "sleeping function called from invalid context".
> 
> kdb's "ftdump" command wants to call ring_buffer_read_prepare() in
> atomic context.  A very simple solution for this is to add allocation
> flags to ring_buffer_read_prepare() so kdb can call it without
> triggering the allocation error.  This patch does that.

I see solution is simple, but now we have a loop with GFP_ATOMIC
allocations inside. How many "tracing spus" is this expected to loop
over? Will not it exhaust atomically available pages and reliably fail
in common configurations?  
								Pavel

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