linux-kernel - Re: [PATCH v12 09/20] dax,ext2: Replace xip_truncate_page with dax_truncate

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Message-Id: <20150112150958.2e6bd85dc3e25b953d28c6cb@linux-foundation.org>
Date:	Mon, 12 Jan 2015 15:09:58 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, willy@...ux.intel.com
Subject: Re: [PATCH v12 09/20] dax,ext2: Replace xip_truncate_page with
 dax_truncate_page

On Fri, 24 Oct 2014 17:20:41 -0400 Matthew Wilcox <matthew.r.wilcox@...el.com> wrote:

> It takes a get_block parameter just like nobh_truncate_page() and
> block_truncate_page()
> 
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -458,3 +458,47 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
>  	return result;
>  }
>  EXPORT_SYMBOL_GPL(dax_fault);
> +
> +/**
> + * dax_truncate_page - handle a partial page being truncated in a DAX file
> + * @inode: The file being truncated
> + * @from: The file offset that is being truncated to
> + * @get_block: The filesystem method used to translate file offsets to blocks
> + *
> + * Similar to block_truncate_page(), this function can be called by a
> + * filesystem when it is truncating an DAX file to handle the partial page.
> + *
> + * We work in terms of PAGE_CACHE_SIZE here for commonality with
> + * block_truncate_page(), but we could go down to PAGE_SIZE if the filesystem
> + * took care of disposing of the unnecessary blocks.

But PAGE_SIZE==PAGE_CACHE_SIZE.  Unclear what you're saying here.

> + Even if the filesystem
> + * block size is smaller than PAGE_SIZE, we have to zero the rest of the page
> + * since the file might be mmaped.
> + */
> 
> ...
>

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