linux-kernel - Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local

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Message-ID: <6a566e51-6288-f782-2fa5-f9b0349b6d7c@redhat.com>
Date:   Tue, 27 Jun 2023 16:34:51 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Sumitra Sharma <sumitraartsy@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Ira Weiny <ira.weiny@...el.com>, Fabio <fmdefrancesco@...il.com>,
        Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()

Hi,

On 6/27/23 15:51, Sumitra Sharma wrote:
> kmap() has been deprecated in favor of the kmap_local_page() due to high
> cost, restricted mapping space, the overhead of a global lock for
> synchronization, and making the process sleep in the absence of free
> slots.
> 
> kmap_local_{page, folio}() is faster than kmap() and offers thread-local
> and CPU-local mappings, can take pagefaults in a local kmap region and
> preserves preemption by saving the mappings of outgoing tasks and
> restoring those of the incoming one during a context switch.
> 
> The difference between kmap_local_page() and kmap_local_folio() consist
> only in the first taking a pointer to a page and the second taking two
> arguments, a pointer to a folio and the byte offset within the folio which
> identifies the page.
> 
> The mappings are kept thread local in the functions 'vboxsf_read_folio',
> 'vboxsf_writepage', 'vboxsf_write_end' in file.c
> 
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>

Thanks, patch looks good to me:

Reviewed-by: Hans de Goede <hdegoede@...hat.com>

Regards,

Hans



> ---
>  fs/vboxsf/file.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/vboxsf/file.c b/fs/vboxsf/file.c
> index 572aa1c43b37..5190619bc3c5 100644
> --- a/fs/vboxsf/file.c
> +++ b/fs/vboxsf/file.c
> @@ -234,7 +234,7 @@ static int vboxsf_read_folio(struct file *file, struct folio *folio)
>  	u8 *buf;
>  	int err;
>  
> -	buf = kmap(page);
> +	buf = kmap_local_folio(folio, off);
>  
>  	err = vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread, buf);
>  	if (err == 0) {
> @@ -245,7 +245,7 @@ static int vboxsf_read_folio(struct file *file, struct folio *folio)
>  		SetPageError(page);
>  	}
>  
> -	kunmap(page);
> +	kunmap_local(buf);
>  	unlock_page(page);
>  	return err;
>  }
> @@ -286,10 +286,10 @@ static int vboxsf_writepage(struct page *page, struct writeback_control *wbc)
>  	if (!sf_handle)
>  		return -EBADF;
>  
> -	buf = kmap(page);
> +	buf = kmap_local_page(page);
>  	err = vboxsf_write(sf_handle->root, sf_handle->handle,
>  			   off, &nwrite, buf);
> -	kunmap(page);
> +	kunmap_local(buf);
>  
>  	kref_put(&sf_handle->refcount, vboxsf_handle_release);
>  
> @@ -320,10 +320,10 @@ static int vboxsf_write_end(struct file *file, struct address_space *mapping,
>  	if (!PageUptodate(page) && copied < len)
>  		zero_user(page, from + copied, len - copied);
>  
> -	buf = kmap(page);
> +	buf = kmap_local_page(page);
>  	err = vboxsf_write(sf_handle->root, sf_handle->handle,
>  			   pos, &nwritten, buf + from);
> -	kunmap(page);
> +	kunmap_local(buf);
>  
>  	if (err) {
>  		nwritten = 0;