Re: [RFC PATCH v3 4/9] mm, fsdax: Refactor memory-failure handler for dax mapping

[Dave Chinner]

On Tue, Dec 15, 2020 at 08:14:09PM +0800, Shiyang Ruan wrote:
> The current memory_failure_dev_pagemap() can only handle single-mapped
> dax page for fsdax mode.  The dax page could be mapped by multiple files
> and offsets if we let reflink feature & fsdax mode work together.  So,
> we refactor current implementation to support handle memory failure on
> each file and offset.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxxxxx>
> ---
.....
>  static const char *action_name[] = {
> @@ -1147,6 +1148,60 @@ static int try_to_split_thp_page(struct page *page, const char *msg)
>  	return 0;
>  }
>  
> +int mf_dax_mapping_kill_procs(struct address_space *mapping, pgoff_t index, int flags)
> +{
> +	const bool unmap_success = true;
> +	unsigned long pfn, size = 0;
> +	struct to_kill *tk;
> +	LIST_HEAD(to_kill);
> +	int rc = -EBUSY;
> +	loff_t start;
> +	dax_entry_t cookie;
> +
> +	/*
> +	 * Prevent the inode from being freed while we are interrogating
> +	 * the address_space, typically this would be handled by
> +	 * lock_page(), but dax pages do not use the page lock. This
> +	 * also prevents changes to the mapping of this pfn until
> +	 * poison signaling is complete.
> +	 */
> +	cookie = dax_lock(mapping, index, &pfn);
> +	if (!cookie)
> +		goto unlock;

Why do we need to prevent the inode from going away here? This
function gets called by XFS after doing an xfs_iget() call to grab
the inode that owns the block. Hence the the inode (and the mapping)
are guaranteed to be referenced and can't go away. Hence for the
filesystem based callers, this whole "dax_lock()" thing can go away.

So, AFAICT, the dax_lock() stuff is only necessary when the
filesystem can't be used to resolve the owner of physical page that
went bad....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx