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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 1 Mar 2024 18:59:28 -0800
Message-ID: <CAHk-=wiBJRgA3iNqihR7uuft=5rog425X_b3uvgroG3fBhktwQ@mail.gmail.com>
Subject: Re: [bug report] dead loop in generic_perform_write() //Re: [PATCH v7
 07/12] iov_iter: Convert iterate*() to inline funcs
To: Tong Tiangen <tongtiangen@huawei.com>, Al Viro <viro@kernel.org>
Cc: David Howells <dhowells@redhat.com>, 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>
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On Thu, 29 Feb 2024 at 09:32, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> One option might be to make a failed memcpy_from_iter_mc() set another
> flag in the iter, and then make fault_in_iov_iter_readable() test that
> flag and return 'len' if that flag is set.
>
> Something like that (wild handwaving) should get the right error handling.
>
> The simpler alternative is maybe something like the attached.
> COMPLETELY UNTESTED. Maybe I've confused myself with all the different
> indiraction mazes in the iov_iter code.

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...

               Linus

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