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SubjectRe: [PATCH] fs: Call kmap_local_page() in copy_string_kernel()
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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
> >
>




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