Re: [PATCH] fs: Call kmap_local_page() in copy_string_kernel()

From: Ira Weiny <ira.weiny@intel.com>
To: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc: Benjamin LaHaise <bcrl@kvack.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Eric Biederman <ebiederm@xmission.com>,
	Kees Cook <keescook@chromium.org>,
	Dan Williams <dan.j.williams@intel.com>,
	"Matthew Wilcox" <willy@infradead.org>, Jan Kara <jack@suse.cz>,
	Jeff Layton <jlayton@kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Jens Axboe <axboe@kernel.dk>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	"Nathan Chancellor" <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Tom Rix <trix@redhat.com>, <linux-aio@kvack.org>,
	<linux-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linux-mm@kvack.org>, <nvdimm@lists.linux.dev>,
	<io-uring@vger.kernel.org>, <linux-riscv@lists.infradead.org>,
	<llvm@lists.linux.dev>
Subject: Re: [PATCH] fs: Call kmap_local_page() in copy_string_kernel()
Date: Thu, 21 Jul 2022 17:14:20 -0700	[thread overview]
Message-ID: <Ytnr3IhSkDOjqbZ1@iweiny-desk3> (raw)
In-Reply-To: <20220710100136.25496-1-fmdefrancesco@gmail.com>

On Sun, Jul 10, 2022 at 12:01:36PM +0200, Fabio M. De Francesco wrote:
> The use of kmap_atomic() is being deprecated in favor of kmap_local_page().
> 
> With kmap_local_page(), the mappings are per thread, CPU local, not
> globally visible and can take page faults. Furthermore, the mappings can be
> acquired from any context (including interrupts).
> 
> Therefore, use kmap_local_page() in copy_string_kernel() instead of
> kmap_atomic().
> 
> Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
> HIGHMEM64GB enabled.
> 
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> ---
> 
> I sent a first patch to fs/exec.c for converting kmap() and kmap_atomic()
> to kmap_local_page():
> https://lore.kernel.org/lkml/20220630163527.9776-1-fmdefrancesco@gmail.com/
> 
> Some days ago, Ira Weiny, while he was reviewing that patch, made me notice
> that I had overlooked a second kmap_atomic() in the same file (thanks):
> https://lore.kernel.org/lkml/YsiQptk19txHrG4c@iweiny-desk3/
> 
> I've been asked to send this as an additional change. This is why there will
> not be any second version of that previous patch.
> 
>  fs/exec.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 4a2129c0d422..5fa652ca5823 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -639,11 +639,11 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
>  		page = get_arg_page(bprm, pos, 1);
>  		if (!page)
>  			return -E2BIG;
> -		kaddr = kmap_atomic(page);
> +		kaddr = kmap_local_page(page);
>  		flush_arg_page(bprm, pos & PAGE_MASK, page);

I really question why we can't use memcpy_to_page() here and move the
flush_arg_page() prior to the mapping?

flush_arg_page() only calls flush_cache_page() which does not need the
mapping to work correctly AFAICT.

Ira

>  		memcpy(kaddr + offset_in_page(pos), arg, bytes_to_copy);
>  		flush_dcache_page(page);
> -		kunmap_atomic(kaddr);
> +		kunmap_local(kaddr);
>  		put_arg_page(page);
>  	}
>  
> -- 
> 2.36.1
> 

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