linux-kernel - Re: [RFC PATCH 01/14] fs/proc/vmcore: Update read_from

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Message-ID: <Ya4nI7ly15pJA5xp@casper.infradead.org>
Date:   Mon, 6 Dec 2021 15:07:15 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Amit Daniel Kachhap <amit.kachhap@....com>,
        linux-kernel@...r.kernel.org,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kexec <kexec@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 01/14] fs/proc/vmcore: Update read_from_oldmem() for
 user pointer

On Mon, Dec 06, 2021 at 03:54:22PM +0100, Christoph Hellwig wrote:
> On Mon, Dec 06, 2021 at 02:17:24PM +0000, Matthew Wilcox wrote:
> > On Mon, Dec 06, 2021 at 03:04:51PM +0100, Christoph Hellwig wrote:
> > > This looks like a huge mess.  What speak against using an iov_iter
> > > here?
> > 
> > I coincidentally made a start on this last night.  Happy to stop.
> 
> Don't stop!
> 
> > What do you think to adding a generic copy_pfn_to_iter()?  Not sure
> > which APIs to use to implement it ... some architectures have weird
> > requirements about which APIs can be used for what kinds of PFNs.
> 
> Hmm.  I though kmap_local_pfn(_prot) is all we need?

In the !HIGHMEM case, that calls pfn_to_page(), and I think the
point of this path is that we don't have a struct page for this pfn.