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From: David Howells <dhowells@redhat.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: dhowells@redhat.com, Tong Tiangen <tongtiangen@huawei.com>,
    Al Viro <viro@kernel.org>, Jens Axboe <axboe@kernel.dk>,
    Christoph Hellwig <hch@lst.de>,
    Christian Brauner <christian@brauner.io>,
    David Laight <David.Laight@aculab.com>,
    Matthew Wilcox <willy@infradead.org>,
    Jeff Layton <jlayton@kernel.org>, linux-fsdevel@vger.kernel.org,
    linux-block@vger.kernel.org, linux-mm@kvack.org,
    netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
    Kefeng Wang <wangkefeng.wang@huawei.com>
Subject: Re: [bug report] dead loop in generic_perform_write() //Re: [PATCH v7 07/12] iov_iter: Convert iterate*() to inline funcs
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Content-ID: <769020.1709553367.1@warthog.procyon.org.uk>
Date: Mon, 04 Mar 2024 11:56:07 +0000
Message-ID: <769021.1709553367@warthog.procyon.org.uk>

Linus Torvalds <torvalds@linux-foundation.org> wrote:

> Actually, I think the right model is to get rid of that horrendous
> .copy_mc field entirely.
> 
> We only have one single place that uses it - that nasty core dumping
> code. And that code is *not* performance critical.
> 
> And not only isn't it performance-critical, it already does all the
> core dumping one page at a time because it doesn't want to write pages
> that were never mapped into user space.
> 
> So what we can do is
> 
>  (a) make the core dumping code *copy* the page to a good location
> with copy_mc_to_kernel() first
> 
>  (b) remove this horrendous .copy_mc crap entirely from iov_iter
> 
> This is slightly complicated by the fact that copy_mc_to_kernel() may
> not even exist, and architectures that don't have it don't want the
> silly extra copy. So we need to abstract the "copy to temporary page"
> code a bit. But that's probably a good thing anyway in that it forces
> us to have nice interfaces.
> 
> End result: something like the attached.
> 
> AGAIN: THIS IS ENTIRELY UNTESTED.
> 
> But hey, so was clearly all the .copy_mc code too that this removes, so...

I like it:-)

I've tested it by SIGQUIT'ing a number of processes and using gdb to examine
the coredumps - which seems to work - at least without the production of any
MCEs.  I'm not sure how I could test it with MCEs.

Feel free to add:

Reviewed-by: David Howells <dhowells@redhat.com>
Tested-by: David Howells <dhowells@redhat.com>

That said, I wonder if:

	#ifdef copy_mc_to_kernel

should be:

	#ifdef CONFIG_ARCH_HAS_COPY_MC

and whether it's possible to find out dynamically if MCEs can occur at all.

David