linux-kernel - Re: [PATCH v1 01/11] fs/proc/vmcore: convert vmcore_cb_lock into vmcore

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Message-ID: <Zz2a5gZq81ZVdFOx@MiWiFi-R3L-srv>
Date: Wed, 20 Nov 2024 16:16:38 +0800
From: Baoquan He <bhe@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-s390@...r.kernel.org, virtualization@...ts.linux.dev,
	kvm@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kexec@...ts.infradead.org, Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>, Dave Young <dyoung@...hat.com>,
	Thomas Huth <thuth@...hat.com>, Cornelia Huck <cohuck@...hat.com>,
	Janosch Frank <frankja@...ux.ibm.com>,
	Claudio Imbrenda <imbrenda@...ux.ibm.com>,
	Eric Farman <farman@...ux.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1 01/11] fs/proc/vmcore: convert vmcore_cb_lock into
 vmcore_mutex

On 11/15/24 at 11:03am, David Hildenbrand wrote:
> On 15.11.24 10:30, Baoquan He wrote:
> > On 10/25/24 at 05:11pm, David Hildenbrand wrote:
> > > We want to protect vmcore modifications from concurrent opening of
> > > the vmcore, and also serialize vmcore modiciations. Let's convert the
> > 
> > 
> > > spinlock into a mutex, because some of the operations we'll be
> > > protecting might sleep (e.g., memory allocations) and might take a bit
> > > longer.
> > 
> > Could you elaborate this a little further. E.g the concurrent opening of
> > vmcore is spot before this patchset or have been seen, and in which place
> > the memory allocation is spot. Asking this becasue I'd like to learn and
> > make clear if this is a existing issue and need be back ported into our
> > old RHEL distros. Thanks in advance.
> 
> It's a preparation for the other patches, that do what is described here:
> 
> a) We can currently modify the vmcore after it was opened. This can happen
> if the vmcoredd is added after the vmcore was loaded. Similar things will
> happen with the PROC_VMCORE_DEVICE_RAM extension.
> 
> b) To handle it cleanly we need to protect the modifications against
> concurrent opening. And the modifcations end up allocating memory and cannot
> easily take the spinlock.
> 
> So far a spinlock was sufficient, now a mutex is required.

I see, as we talked in patch 2 sub-thread, these information are very
valuable to help people get the background information when they read
code. Let's put it in patch log. Thanks.

> 
> Maybe we'd want to backport 1,2,3, but not sure if we consider this critical
> enough.
> 
> -- 
> Cheers,
> 
> David / dhildenb
>