Re: [PATCH] [RFC] mm: migrate: rcu stalls because of invalid swap cache entries

[Charan Teja Kalla <quic_charante@quicinc.com>]

Thanks Matthew!

On 11/21/2023 9:43 PM, Matthew Wilcox wrote:
> What would you think to this?  I think a better fix would be to
> fix the swap cache to user multi-order entries, but I would like to
> see this backportable!
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index d9d2b9432e81..2d67ca47d2e2 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -405,6 +405,7 @@ int folio_migrate_mapping(struct address_space *mapping,
>  	int dirty;
>  	int expected_count = folio_expected_refs(mapping, folio) + extra_count;
>  	long nr = folio_nr_pages(folio);
> +	long entries, i;
>  
>  	if (!mapping) {
>  		/* Anonymous page without mapping */
> @@ -442,8 +443,10 @@ int folio_migrate_mapping(struct address_space *mapping,
>  			folio_set_swapcache(newfolio);
>  			newfolio->private = folio_get_private(folio);
>  		}
> +		entries = nr;
>  	} else {
>  		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
> +		entries = 1;
>  	}
>  
>  	/* Move dirty while page refs frozen and newpage not yet exposed */
> @@ -453,7 +456,11 @@ int folio_migrate_mapping(struct address_space *mapping,
>  		folio_set_dirty(newfolio);
>  	}
>  
> -	xas_store(&xas, newfolio);
> +	/* Swap cache still stores N entries instead of a high-order entry */
> +	for (i = 0; i < entries; i++) {
> +		xas_store(&xas, newfolio);
> +		xas_next(&xas);
> +	}
>  
>  	/*
>  	 * Drop cache reference from old page by unfreezing
Seems a cleaner one to store N entries. Supporting swap cache for multi
order entries might be time consuming. Till then, can we use this patch
as the solution, with the proper commit log conveying revert of this
patch when swap cache supported with Multi-order indices?

Please Lmk, If I can raise this patch with suggested-by:you .

Thanks
Charan