Re: [PATCH v2 1/1] iomap: propagate nowait to block layer

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

On 3/4/25 16:07, Christoph Hellwig wrote:
> On Tue, Mar 04, 2025 at 12:18:07PM +0000, Pavel Begunkov wrote:
>>   	    ((dio->flags & IOMAP_DIO_NEED_SYNC) && !use_fua) ||
>> -	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode)))
>> +	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode))) {
>>   		dio->flags &= ~IOMAP_DIO_CALLER_COMP;
>>   
>> +		if (!is_sync_kiocb(dio->iocb) &&
>> +		    (dio->iocb->ki_flags & IOCB_NOWAIT))
>> +			return -EAGAIN;
> 
> Black magic without comments explaining it.

I can copy the comment from below if you wish.

>> +	if (!is_sync_kiocb(dio->iocb) && (dio->iocb->ki_flags & IOCB_NOWAIT)) {
>> +		/*
>> +		 * This is nonblocking IO, and we might need to allocate
>> +		 * multiple bios. In this case, as we cannot guarantee that
>> +		 * one of the sub bios will not fail getting issued FOR NOWAIT
>> +		 * and as error results are coalesced across all of them, ask
>> +		 * for a retry of this from blocking context.
>> +		 */
>> +		if (bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_VECS + 1) >
>> +					  BIO_MAX_VECS)
> 
> This is not very accurate in times of multi-page bvecs and large order
> folios all over.

bio_iov_vecs_to_alloc() can overestimate, i.e. the check might return
-EAGAIN in more cases than required but not the other way around,
that should be enough for a fix such as this patch. Or did I maybe
misunderstood you?

> I think you really need to byte the bullet and support for early returns
> from the non-blocking bio submission path.

Assuming you're suggesting to implement that, I can't say I'm excited by
the idea of reworking a non trivial chunk of block layer to fix a problem
and then porting it up to some 5.x, especially since it was already
attempted before by someone and ultimately got reverted.

-- 
Pavel Begunkov