Re: [RFC PATCH] mm: move xa forward when run across zombie page

[Matthew Wilcox]

On Fri, Oct 14, 2022 at 01:30:48PM +0800, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> 
> Bellowing RCU stall is reported where kswapd traps in a live lock when shrink
> superblock's inode list. The direct reason is zombie page keeps staying on the
> xarray's slot and make the check and retry loop permanently. The root cause is unknown yet
> and supposed could be an xa update without synchronize_rcu etc. I would like to
> suggest skip this page to break the live lock as a workaround.

No, the underlying bug should be fixed.

>  	if (!folio || xa_is_value(folio))
>  		return folio;
>  
> -	if (!folio_try_get_rcu(folio))
> +	if (!folio_try_get_rcu(folio)) {
> +		xas_advance(xas, folio->index + folio_nr_pages(folio) - 1);
>  		goto reset;
> +	}

You can't do this anyway.  To call folio_nr_pages() and to look at
folio->index, you must have a refcount on the page, and this is the
path where we failed to get the refcount.