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

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

HI Matthew, Just a ping to have your valuable opinion here.

Thanks,
Charan

On 11/23/2023 7:55 PM, Charan Teja Kalla 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