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

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Message-ID: <20210618030157.GA1905674@iweiny-DESK2.sc.intel.com>
Date:   Thu, 17 Jun 2021 20:01:57 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 01/18] mm: add a kunmap_local_dirty helper

On Tue, Jun 15, 2021 at 03:24:39PM +0200, Christoph Hellwig wrote:
> Add a helper that calls flush_kernel_dcache_page before unmapping the
> local mapping.  flush_kernel_dcache_page is required for all pages
> potentially mapped into userspace that were written to using kmap*,
> so having a helper that does the right thing can be very convenient.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  include/linux/highmem-internal.h | 7 +++++++
>  include/linux/highmem.h          | 4 ++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
> index 7902c7d8b55f..bd37706db147 100644
> --- a/include/linux/highmem-internal.h
> +++ b/include/linux/highmem-internal.h
> @@ -224,4 +224,11 @@ do {								\
>  	__kunmap_local(__addr);					\
>  } while (0)
>  
> +#define kunmap_local_dirty(__page, __addr)			\

I think having to store the page and addr to return to kunmap_local_dirty() is
going to be a pain in some code paths.  Not a show stopper but see below...

> +do {								\
> +	if (!PageSlab(__page))					\

Was there some clarification why the page can't be a Slab page?  Or is this
just an optimization?

> +		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.

Ira

[1] https://lore.kernel.org/lkml/20210615050258.GA5208@gondor.apana.org.au/

> +	kunmap_local(__addr);					\
> +} while (0)
> +
>  #endif
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 832b49b50c7b..65f548db4f2d 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -93,6 +93,10 @@ static inline void kmap_flush_unused(void);
>   * On HIGHMEM enabled systems mapping a highmem page has the side effect of
>   * disabling migration in order to keep the virtual address stable across
>   * preemption. No caller of kmap_local_page() can rely on this side effect.
> + *
> + * If data is written to the returned kernel mapping, the callers needs to
> + * unmap the mapping using kunmap_local_dirty(), else kunmap_local() should
> + * be used.
>   */
>  static inline void *kmap_local_page(struct page *page);
>  
> -- 
> 2.30.2
>