linux-kernel - Re: [PATCH] module: Replace kmap() with kmap_local

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Date:   Mon, 18 Jul 2022 15:19:50 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Song Liu <song@...nel.org>, Takashi Iwai <tiwai@...e.de>,
        Adam Manzanares <a.manzanares@...sung.com>,
        Davidlohr Bueso <dave@...olabs.net>
Cc:     linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] module: Replace kmap() with kmap_local_page()

On Mon, Jul 18, 2022 at 02:26:45AM +0200, Fabio M. De Francesco wrote:
> kmap() is being deprecated in favor of kmap_local_page().
> 
> 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.

Neat!

> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).

Yes but the mapping can only be accessed from within this thread and the
thread is bound to the CPU while the mapping is active. So, what if we
get a preemption during decompression here? What happens?

> It is faster than kmap() in kernels with HIGHMEM enabled. Its use in
> module_gzip_decompress() and in module_xz_decompress() is safe and
> should be preferred.
> 
> Therefore, replace kmap() with kmap_local_page().

While this churn is going on everywhere I was wondering why not
go ahead and adopt kmap_local_folio() instead?

  Luis

> Tested on a QEMU/KVM x86_32 VM with 4GB RAM, booting kernels with
> HIGHMEM64GB enabled. Modules compressed with XZ or GZIP decompress
> properly.
> 
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
>  kernel/module/decompress.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/module/decompress.c b/kernel/module/decompress.c
> index 2fc7081dd7c1..4d0bcb3d9e44 100644
> --- a/kernel/module/decompress.c
> +++ b/kernel/module/decompress.c
> @@ -119,10 +119,10 @@ static ssize_t module_gzip_decompress(struct load_info *info,
>  			goto out_inflate_end;
>  		}
>  
> -		s.next_out = kmap(page);
> +		s.next_out = kmap_local_page(page);
>  		s.avail_out = PAGE_SIZE;
>  		rc = zlib_inflate(&s, 0);
> -		kunmap(page);
> +		kunmap_local(s.next_out);
>  
>  		new_size += PAGE_SIZE - s.avail_out;
>  	} while (rc == Z_OK);
> @@ -178,11 +178,11 @@ static ssize_t module_xz_decompress(struct load_info *info,
>  			goto out;
>  		}
>  
> -		xz_buf.out = kmap(page);
> +		xz_buf.out = kmap_local_page(page);
>  		xz_buf.out_pos = 0;
>  		xz_buf.out_size = PAGE_SIZE;
>  		xz_ret = xz_dec_run(xz_dec, &xz_buf);
> -		kunmap(page);
> +		kunmap_local(xz_buf.out);
>  
>  		new_size += xz_buf.out_pos;
>  	} while (xz_buf.out_pos == PAGE_SIZE && xz_ret == XZ_OK);
> -- 
> 2.37.1
>