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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>,
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        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>,
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        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
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        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, linux-aio@kvack.org,
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        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
Message-ID: <2115146.irdbgypaU6@opensuse>
In-Reply-To: <Ytnr3IhSkDOjqbZ1@iweiny-desk3>
References: <20220710100136.25496-1-fmdefrancesco@gmail.com> <Ytnr3IhSkDOjqbZ1@iweiny-desk3>
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On venerd=C3=AC 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=20
kmap_local_page().
> >=20
> > With kmap_local_page(), the mappings are per thread, CPU local, not
> > globally visible and can take page faults. Furthermore, the mappings=20
can be
> > acquired from any context (including interrupts).
> >=20
> > Therefore, use kmap_local_page() in copy_string_kernel() instead of
> > kmap_atomic().
> >=20
> > Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
> > HIGHMEM64GB enabled.
> >=20
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > ---
> >=20
> > I sent a first patch to fs/exec.c for converting kmap() and=20
kmap_atomic()
> > to kmap_local_page():
> > https://lore.kernel.org/lkml/20220630163527.9776-1-fmdefrancesco@gmail.=
com/
> >=20
> > Some days ago, Ira Weiny, while he was reviewing that patch, made me=20
notice
> > that I had overlooked a second kmap_atomic() in the same file (thanks):
> > https://lore.kernel.org/lkml/YsiQptk19txHrG4c@iweiny-desk3/
> >=20
> > I've been asked to send this as an additional change. This is why there=
=20
will
> > not be any second version of that previous patch.
> >=20
> >  fs/exec.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >=20
> > 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=20
linux_binprm *bprm)
> >  		page =3D get_arg_page(bprm, pos, 1);
> >  		if (!page)
> >  			return -E2BIG;
> > -		kaddr =3D kmap_atomic(page);
> > +		kaddr =3D kmap_local_page(page);
> >  		flush_arg_page(bprm, pos & PAGE_MASK, page);
>=20
> I really question why we can't use memcpy_to_page() here and move the
> flush_arg_page() prior to the mapping?
>=20
> 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=20
flush_arg_page() does not need to be called while the task holds the local=
=20
mapping :-(

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

Thanks for your comment,

=46abio

>=20
> Ira
>=20
> >  		memcpy(kaddr + offset_in_page(pos), arg,=20
bytes_to_copy);
> >  		flush_dcache_page(page);
> > -		kunmap_atomic(kaddr);
> > +		kunmap_local(kaddr);
> >  		put_arg_page(page);
> >  	}
> > =20
> > --=20
> > 2.36.1
> >=20
>=20