Re: [PATCH for-next] io_uring: let to set a range for file slot allocation

[Pavel Begunkov <asml.silence@gmail.com>]

On 6/27/22 08:47, Hao Xu wrote:
> On 6/25/22 18:55, Pavel Begunkov wrote:
>>  From recently io_uring provides an option to allocate a file index for
>> operation registering fixed files. However, it's utterly unusable with
>> mixed approaches when for a part of files the userspace knows better
>> where to place it, as it may race and users don't have any sane way to
>> pick a slot and hoping it will not be taken.
> 
> Exactly, with high frequency of index allocation from like multishot
> accept, it's easy that user-pick requests fails.
> By the way, just curious, I can't recall a reason that users pick a slot
> rather than letting kernel do the decision, is there any? So I guess

Can't say for the initial design, but I prefer to give away control
over such stuff to the userspace 1) to not over pollute the kernel
(not relevant anymore), 2) because it has more knowledge and can
use it more efficiently. E.g. to have O(1) memory and search time
by using in-place index based free slot list, when indexes can be
contants, and so on.


> users may use all the indexes as 'file slot allocation' range. Correct
> me if I miss something.

Yeah, can be enough, and that's what the range is set to by default.

>> Let the userspace to register a range of fixed file slots in which the
>> auto-allocation happens. The use case is splittting the fixed table in
>> two parts, where on of them is used for auto-allocation and another for
>> slot-specified operations.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
[...]
>> @@ -24,11 +24,10 @@ static int io_file_bitmap_get(struct io_ring_ctx *ctx)
>>           if (ret != nr)
>>               return ret;
>> -        if (!table->alloc_hint)
>> +        if (table->alloc_hint == ctx->file_alloc_start)
>>               break;
>> -
>>           nr = table->alloc_hint;
>> -        table->alloc_hint = 0;
>> +        table->alloc_hint = ctx->file_alloc_start;
> 
> should we use io_reset_alloc_hint() ?

We could but I'd rather prefer not. It's used just to anything valid
within the range, while in io_file_bitmap_get() it's specifically to
wrap around.

-- 
Pavel Begunkov