linux-kernel - Re: [PATCH v2] fs: Replace kmap{,_atomic}() with kmap_local

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Date:   Tue, 16 Aug 2022 12:08:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH v2] fs: Replace kmap{,_atomic}() with kmap_local_page()

On Sat, Aug 13, 2022 at 03:36:53PM +0200, Fabio M. De Francesco wrote:
> On mercoledì 3 agosto 2022 20:28:56 CEST Fabio M. De Francesco wrote:
> > The use of kmap() and kmap_atomic() are being deprecated in favor of
> > kmap_local_page().
> > 
> > There are two main problems with kmap(): (1) It comes with an overhead as
> > mapping space is restricted and protected by a global lock for
> > synchronization and (2) it also requires global TLB invalidation when the
> > kmap’s pool wraps and it might block when the mapping space is fully
> > utilized until a slot becomes available.
> > 
> > With kmap_local_page() the mappings are per thread, CPU local, can take
> > page faults, and can be called from any context (including interrupts).
> > It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> > the tasks can be preempted and, when they are scheduled to run again, the
> > kernel virtual addresses are restored and are still valid.
> > 
> > Since the use of kmap_local_page() in exec.c is safe, it should be
> > preferred everywhere in exec.c.
> > 
> > As said, since kmap_local_page() can be also called from atomic context,
> > and since remove_arg_zero() doesn't (and shouldn't ever) rely on an
> > implicit preempt_disable(), this function can also safely replace
> > kmap_atomic().
> > 
> > Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in
> > fs/exec.c.
> > 
> > Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel
> > with HIGHMEM64GB enabled.
> > 
> > Cc: Eric W. Biederman <ebiederm@...ssion.com>
> > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> > Reviewed-by: Ira Weiny <ira.weiny@...el.com>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> > ---
> > 
> > v1->v2: Added more information to the commit log to address some
> > objections expressed by Eric W. Biederman[1] in reply to v1. No changes
> > have been made to the code. Forwarded a tag from Ira Weiny (thanks!).
> > 
> > [1]
> > https://lore.kernel.org/lkml/8735fmqcfz.fsf@email.froward.int.ebiederm.org/
> >
> >  fs/exec.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> > 
> 
> Hi Kees,
> 
> After that thread about the report from Syzbot, and the subsequent discussion, 
> I noticed that you didn't yet take this other patch for exec.c.
> 
> I suppose that the two patches would better go out together. So I'm writing 
> for sending a gentle ping.
> 
> As I said, no changes have been made to the code with respect to v1 (which I 
> submitted in June). However, later I thought that adding more information 
> might have helped reviewers and maintainers to better understand the why of 
> this patch.

Oops, thanks for the ping. I'll pull this now.

-- 
Kees Cook