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

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Date:   Thu, 30 Jun 2022 12:38:08 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Benjamin LaHaise <bcrl@...ck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, nvdimm@...ts.linux.dev,
        io-uring@...r.kernel.org, linux-riscv@...ts.infradead.org,
        llvm@...ts.linux.dev, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] fs: Replace kmap{,_atomic}() with kmap_local_page()

"Fabio M. De Francesco" <fmdefrancesco@...il.com> writes:

> The use of kmap() and kmap_atomic() are being deprecated in favor of
> kmap_local_page().
>
> With kmap_local_page(), the mappings are per thread, CPU local and not
> globally visible. Furthermore, the mappings can be acquired from any
> context (including interrupts).
>
> Therefore, use kmap_local_page() in exec.c because these mappings are per
> thread, CPU local, and not globally visible.
>
> Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
> HIGHMEM64GB enabled.

Can someone please refresh my memory on what is going on.

I remember there were limitations that kmap_atomic had that are hard to
meet so something I think it was kmap_local was invented and created
to be the kmap_atomic replacement.

What are the requirements on kmap_local?  In copy_strings
kmap is called in contexts that can sleep in page faults so any
nearly any requirement except a thread local use is invalidated.

As you have described kmap_local above it does not sound like kmap_local
is safe in this context, but that could just be a problem in description
that my poor memory does is not recalling the necessary details to
correct.

Eric

> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
>  fs/exec.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 0989fb8472a1..4a2129c0d422 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -583,11 +583,11 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>  
>  				if (kmapped_page) {
>  					flush_dcache_page(kmapped_page);
> -					kunmap(kmapped_page);
> +					kunmap_local(kaddr);
>  					put_arg_page(kmapped_page);
>  				}
>  				kmapped_page = page;
> -				kaddr = kmap(kmapped_page);
> +				kaddr = kmap_local_page(kmapped_page);
>  				kpos = pos & PAGE_MASK;
>  				flush_arg_page(bprm, kpos, kmapped_page);
>  			}
> @@ -601,7 +601,7 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>  out:
>  	if (kmapped_page) {
>  		flush_dcache_page(kmapped_page);
> -		kunmap(kmapped_page);
> +		kunmap_local(kaddr);
>  		put_arg_page(kmapped_page);
>  	}
>  	return ret;
> @@ -883,11 +883,11 @@ int transfer_args_to_stack(struct linux_binprm *bprm,
>  
>  	for (index = MAX_ARG_PAGES - 1; index >= stop; index--) {
>  		unsigned int offset = index == stop ? bprm->p & ~PAGE_MASK : 0;
> -		char *src = kmap(bprm->page[index]) + offset;
> +		char *src = kmap_local_page(bprm->page[index]) + offset;
>  		sp -= PAGE_SIZE - offset;
>  		if (copy_to_user((void *) sp, src, PAGE_SIZE - offset) != 0)
>  			ret = -EFAULT;
> -		kunmap(bprm->page[index]);
> +		kunmap_local(src);
>  		if (ret)
>  			goto out;
>  	}
> @@ -1680,13 +1680,13 @@ int remove_arg_zero(struct linux_binprm *bprm)
>  			ret = -EFAULT;
>  			goto out;
>  		}
> -		kaddr = kmap_atomic(page);
> +		kaddr = kmap_local_page(page);
>  
>  		for (; offset < PAGE_SIZE && kaddr[offset];
>  				offset++, bprm->p++)
>  			;
>  
> -		kunmap_atomic(kaddr);
> +		kunmap_local(kaddr);
>  		put_arg_page(page);
>  	} while (offset == PAGE_SIZE);