linux-kernel - Re: [PATCH 01/18] mm: add a kunmap_local

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Date:   Fri, 18 Jun 2021 11:12:58 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Geoff Levand <geoff@...radead.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Dongsheng Yang <dongsheng.yang@...ystack.cn>,
        Mike Snitzer <snitzer@...hat.com>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        dm-devel@...hat.com, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, ceph-devel@...r.kernel.org,
        linux-arch@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Christoph Lameter <cl@...two.de>
Subject: Re: [PATCH 01/18] mm: add a kunmap_local_dirty helper

On Fri, Jun 18, 2021 at 11:37:28AM +0800, Herbert Xu wrote:
> On Thu, Jun 17, 2021 at 08:01:57PM -0700, Ira Weiny wrote:
> >
> > > +		flush_kernel_dcache_page(__page);		\
> > 
> > Is this required on 32bit systems?  Why is kunmap_flush_on_unmap() not
> > sufficient on 64bit systems?  The normal kunmap_local() path does that.
> > 
> > I'm sorry but I did not see a conclusion to my query on V1. Herbert implied the
> > he just copied from the crypto code.[1]  I'm concerned that this _dirty() call
> > is just going to confuse the users of kmap even more.  So why can't we get to
> > the bottom of why flush_kernel_dcache_page() needs so much logic around it
> > before complicating the general kernel users.
> > 
> > I would like to see it go away if possible.
> 
> This thread may be related:
> 
> https://lwn.net/Articles/240249/

Interesting!  Thanks!

Digging around a bit more I found:

https://lore.kernel.org/patchwork/patch/439637/

Auditing all the flush_dcache_page() arch code reveals that the mapping field
is either unused, or is checked for NULL.  Furthermore, all the implementations
call page_mapping_file() which further limits the page to not be a swap page.

All flush_kernel_dcache_page() implementations appears to operate the same way
in all arch's which define that call.

So I'm confident now that additional !PageSlab(__page) checks are not needed
and this patch is unnecessary.   Christoph, can we leave this out of the kmap
API and just fold the flush_kernel_dcache_page() calls back into the bvec code?

Unfortunately, I'm not convinced this can be handled completely by
kunmap_local() nor the mem*_page() calls because there is a difference between
flush_dcache_page() and flush_kernel_dcache_page() in most archs...  [parisc
being an exception which falls back to flush_kernel_dcache_page()]...

It seems like the generic unmap path _should_ be able to determine which call
to make based on the page but I'd have to look at that more.

Ira