linux-kernel - Re: [PATCH 5.18 112/181] vmcore: convert copy_oldmem_page() to take an iov

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Date:   Tue, 5 Jul 2022 13:53:58 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Baoquan He <bhe@...hat.com>, Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH 5.18 112/181] vmcore: convert copy_oldmem_page() to take
 an iov_iter

On Mon, Jul 04, 2022 at 06:26:08PM +0100, Matthew Wilcox wrote:

Hi Matthew,

> > > > cc02e6e21aa5 ("s390/crash: add missing iterator advance in copy_oldmem_page()")
> > > > af2debd58bd7 ("s390/crash: make copy_oldmem_page() return number of bytes copied")
> > > 
> > > Both of them are also in the 5.18-rc queue here, right?
> > 
> > Yes, these are:
> > 
> > 	[PATCH 5.18 113/181] s390/crash: add missing iterator advance in copy_oldmem_page() Greg Kroah-Hartman
> 
> It's generally considered polite to cc the original author when you
> fix one of their patches.  I wasn't aware of this patch.

Apologies for not doing that - I did not realize this patch could be
of interest for non-s390.

> While the code change looks right, the commit message is wrong;
> copy_oldmem_user() and copy_oldmem_kernel() need to GO AWAY.  You
> need to be more like the other architectures and end up calling
> copy_to_iter().  I have no idea what this memcpy_hsa_kernel()
> and memcpy_hsa_user() are all about, but I was hoping that somebody
> from the s390 team would react to:
> 
>     s390 needs more work to pass the iov_iter down further, or refactor, but
>     I'd be more comfortable if someone who can test on s390 did that work.
> 
> Maybe you'll do it.

I considered going with copy_to_iter(), but unfortunately getting rid of
copy_oldmem_user() and copy_oldmem_kernel() is not an easy thing to do,
if possible. At least for the time being we have to stay with these two
and handle copy_oldmem_page() on our own.

Yet, a hope that a single-segment iterator on s390 would be enough
turned out to be wrong and a follow-up fix is coming. Hopefully, it
will make s390 code one step closer to others.

Thanks!