linux-kernel - Re: [PATCH v2 0/5] kdump: crashkernel reservation from CMA

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Message-ID: <aDmDNdhW75eXw_4M@dwarf.suse.cz>
Date: Fri, 30 May 2025 12:06:45 +0200
From: Jiri Bohac <jbohac@...e.cz>
To: David Hildenbrand <david@...hat.com>
Cc: Michal Hocko <mhocko@...e.com>, Baoquan He <bhe@...hat.com>,
	Donald Dutile <ddutile@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
	Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
	Philipp Rudo <prudo@...hat.com>, Pingfan Liu <piliu@...hat.com>,
	Tao Liu <ltao@...hat.com>, linux-kernel@...r.kernel.org,
	David Hildenbrand <dhildenb@...hat.com>
Subject: Re: [PATCH v2 0/5] kdump: crashkernel reservation from CMA

On Fri, May 30, 2025 at 11:47:46AM +0200, David Hildenbrand wrote:
> > > > crashkernel=1G,cma,cma_sane_dma # no wait on transition
> > > 
> > > But is no wait ok? I mean, any O_DIRECT with any device would at least take
> > > a bit, no?
> > > 
> > > Of course, there is a short time between the crash and actually triggerying
> > > kdump.
> > > 
> > > > crashkernel=1G,cma # wait on transition with e.g. 10s timeout
> > > 
> > > In general, would work for me.
> > 
> > I don't like extending the crashkernel= syntax like this.
> > It would make hooking into the generic parsing code in
> > parse_crashkernel() really ugly. The syntax is already
> > convoluted as is and hard enough to explain in the documentation.
> 
> Would another boolean flag (on top of the other one you are adding) really
> make this significantly more ugly?

the current code does not split the parameter by commas and treat
the part as boolean flags.

Both ",cma" and ",cma,cma_sane_dma" (and possibly
",cma_sane_dma,cma") would need to be added to suffix_tbl[]
(carefully thinking about the order because one is a prefix of the
other); then handled almost the same except setting the flag.

Also I think using the command line is way less flexible than
sysfs. E.g. the userspace tool loading the crash kernel (kdump)
may want to decide if the hardware is sane using its own
whitelist/blacklist...

> > I am inclined to just setting the fixed delay to 10s for now and
> > adding a sysfs knob later if someone asks for it.
> > 
> > Would that work for you?
> 
> Sure. We could always add such a flag later if it's really a problem for
> someone.

OK, thanks! Will post the v4 shortly.

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, Prague, Czechia