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

From: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To: Ira Weiny <ira.weiny@intel.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: Sat, 23 Jul 2022 03:02:32 +0200	[thread overview]
Message-ID: <2115146.irdbgypaU6@opensuse> (raw)
In-Reply-To: <Ytnr3IhSkDOjqbZ1@iweiny-desk3>

On venerdì 22 luglio 2022 02:14:20 CEST Ira Weiny wrote:
> 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.

You're right here. I'm sorry for being so lazy and not checking that 
flush_arg_page() does not need to be called while the task holds the local 
mapping :-(

In v2 I'll move flush_arg_page() one line above memcpy_to_page().

Thanks for your comment,

Fabio

> 
> 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
> > 
> 