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
