Re: [PATCH -V5 RESEND 03/21] swap: Support PMD swap mapping in swap_duplicate()

[Huang\, Ying]

Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:

> On Tue, Sep 25, 2018 at 03:13:30PM +0800, Huang Ying wrote:
>> @@ -3487,35 +3521,66 @@ static int __swap_duplicate_locked(struct swap_info_struct *p,
>>  }
>>  
>>  /*
>> - * Verify that a swap entry is valid and increment its swap map count.
>> + * Verify that the swap entries from *entry is valid and increment their
>> + * PMD/PTE swap mapping count.
>>   *
>>   * Returns error code in following case.
>>   * - success -> 0
>>   * - swp_entry is invalid -> EINVAL
>> - * - swp_entry is migration entry -> EINVAL
>
> I'm assuming it wasn't possible to hit this error before this patch, and you're
> just removing it now since you're in the area?

Yes.

>>   * - swap-cache reference is requested but there is already one. -> EEXIST
>>   * - swap-cache reference is requested but the entry is not used. -> ENOENT
>>   * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
>> + * - the huge swap cluster has been split. -> ENOTDIR
>
> Strangely intuitive choice of error code :)

Thanks!  It doesn't match the error exactly, but I have no better choice
now.  Matthew Wilcox have suggested to use an swap specific enum
instead.  I think that is good in general, but we need only one extra
error code, and we need to change the interface of several swap
functions.  So I think that should be in a separate patchset if
necessary.

>>  /*
>>   * Increase reference count of swap entry by 1.
>> - * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
>> - * but could not be atomically allocated.  Returns 0, just as if it succeeded,
>> - * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
>> - * might occur if a page table entry has got corrupted.
>> + *
>> + * Return error code in following case.
>> + * - success -> 0
>> + * - swap_count_continuation is required but could not be atomically allocated.
>> + *   *entry is used to return swap entry to call add_swap_count_continuation().
>> + *								      -> ENOMEM
>> + * - otherwise same as __swap_duplicate()
>>   */
>> -int swap_duplicate(swp_entry_t entry)
>> +int swap_duplicate(swp_entry_t *entry, int entry_size)
>>  {
>>  	int err = 0;
>>  
>> -	while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
>> -		err = add_swap_count_continuation(entry, GFP_ATOMIC);
>> +	while (!err &&
>> +	       (err = __swap_duplicate(entry, entry_size, 1)) == -ENOMEM)
>> +		err = add_swap_count_continuation(*entry, GFP_ATOMIC);
>>  	return err;
>
> Now we're returning any error we get from __swap_duplicate, apparently to
> accommodate ENOTDIR later in the series, which is a change from the behavior
> introduced in 570a335b8e22 ("swap_info: swap count continuations").  This might
> belong in a separate patch given its potential for side effects.

I have checked all the calls of the function and found there will be no
bad effect.  Do you have any side effect?

> Although, I don't understand why 570a335b8e22 ignored errors other than -ENOMEM
> when both swap_duplicate callers _seem_ from a quick read to be able to respond
> gracefully to any error.

Before 570a335b8e22, all errors are ignored in swap_duplicate() (its
type is void).  If my understanding were correct, all errors except
-ENOMEM are impossible before changes in this patchset.  So they are
ignored.

Best Regards,
Huang, Ying