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

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Message-ID: <aDiJuZgUC2CYtdxi@dwarf.suse.cz>
Date: Thu, 29 May 2025 18:22:17 +0200
From: Jiri Bohac <jbohac@...e.cz>
To: David Hildenbrand <david@...hat.com>
Cc: 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>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH v2 0/5] kdump: crashkernel reservation from CMA

On Wed, May 28, 2025 at 11:01:04PM +0200, David Hildenbrand wrote:
> I think we just have to be careful to document it properly -- especially the
> shortcomings and that this feature might become a problem in the future.
> Movable user-space page tables getting placed on CMA memory would probably
> not be a problem if we don't care about ... user-space data either way.

Agreed; in the v3 series [1] I amended the documentation part [2] to
explicitly mention that kernel movable allocations could be
missing from the vmcore.

The risks associated with pending DMA are also mentioned.

Is there anything you're still missing from the v3 documentation?

> The whole "Direct I/O takes max 1s" part is a bit shaky. Maybe it could be
> configurable how long to wait? 10s is certainly "safer".

I have nothing against making this configurable, or just setting
the fixed/default delay to 10s. Which would you prefer?
Would you prefer a command-line option, config option or a sysfs
file?

Thanks!

[1] https://lore.kernel.org/lkml/Z9H10pYIFLBHNKpr@dwarf.suse.cz/
[2] https://lore.kernel.org/lkml/Z9H4E82EslkGR7pV@dwarf.suse.cz/

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