linux-kernel - Re: [PATCH v6 05/26] fs/dax: Create a common implementation to break DAX layouts

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Message-ID: <6785ada48f85_20f3294e1@dwillia2-xfh.jf.intel.com.notmuch>
Date: Mon, 13 Jan 2025 16:19:48 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Alistair Popple <apopple@...dia.com>, <akpm@...ux-foundation.org>,
	<dan.j.williams@...el.com>, <linux-mm@...ck.org>
CC: <alison.schofield@...el.com>, Alistair Popple <apopple@...dia.com>,
	<lina@...hilina.net>, <zhang.lyra@...il.com>,
	<gerald.schaefer@...ux.ibm.com>, <vishal.l.verma@...el.com>,
	<dave.jiang@...el.com>, <logang@...tatee.com>, <bhelgaas@...gle.com>,
	<jack@...e.cz>, <jgg@...pe.ca>, <catalin.marinas@....com>, <will@...nel.org>,
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	<linux-cxl@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
	<jhubbard@...dia.com>, <hch@....de>, <david@...morbit.com>,
	<chenhuacai@...nel.org>, <kernel@...0n.name>, <loongarch@...ts.linux.dev>
Subject: Re: [PATCH v6 05/26] fs/dax: Create a common implementation to break
 DAX layouts

Alistair Popple wrote:
> Prior to freeing a block file systems supporting FS DAX must check
> that the associated pages are both unmapped from user-space and not
> undergoing DMA or other access from eg. get_user_pages(). This is
> achieved by unmapping the file range and scanning the FS DAX
> page-cache to see if any pages within the mapping have an elevated
> refcount.
> 
> This is done using two functions - dax_layout_busy_page_range() which
> returns a page to wait for the refcount to become idle on. Rather than
> open-code this introduce a common implementation to both unmap and
> wait for the page to become idle.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> 
> ---
> 
> Changes for v5:
> 
>  - Don't wait for idle pages on non-DAX mappings
> 
> Changes for v4:
> 
>  - Fixed some build breakage due to missing symbol exports reported by
>    John Hubbard (thanks!).
[..]
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index cc1acb1..ee8e83f 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3917,15 +3917,7 @@ int ext4_break_layouts(struct inode *inode)
>  	if (WARN_ON_ONCE(!rwsem_is_locked(&inode->i_mapping->invalidate_lock)))
>  		return -EINVAL;
>  
> -	do {
> -		page = dax_layout_busy_page(inode->i_mapping);
> -		if (!page)
> -			return 0;
> -
> -		error = dax_wait_page_idle(page, ext4_wait_dax_page, inode);
> -	} while (error == 0);
> -
> -	return error;
> +	return dax_break_mapping_inode(inode, ext4_wait_dax_page);

I hit this in my compile testing:

fs/ext4/inode.c: In function ‘ext4_break_layouts’:
fs/ext4/inode.c:3915:13: error: unused variable ‘error’ [-Werror=unused-variable]
 3915 |         int error;
      |             ^~~~~
fs/ext4/inode.c:3914:22: error: unused variable ‘page’ [-Werror=unused-variable]
 3914 |         struct page *page;
      |                      ^~~~
cc1: all warnings being treated as errors

...which gets fixed up later on, but bisect breakage is unwanted.

The bots will probably find this too eventually.