Re: io_uring force_nonblock vs POSIX_FADV_WILLNEED

[Jens Axboe <axboe@kernel.dk>]

On 2/2/20 12:14 AM, Andres Freund wrote:
> Hi,
> 
> On 2020-02-01 09:22:45 -0700, Jens Axboe wrote:
>> On 2/1/20 2:43 AM, Andres Freund wrote:
>>> Seems like either WILLNEED would have to always be deferred, or
>>> force_page_cache_readahead, __do_page_cache_readahead would etc need to
>>> be wired up to know not to block. Including returning EAGAIN, despite
>>> force_page_cache_readahead and generic_readahead() intentially ignoring
>>> return values / errors.
>>>
>>> I guess it's also possible to just add a separate precheck that looks
>>> whether there's any IO needing to be done for the range. That could
>>> potentially also be used to make DONTNEED nonblocking in case everything
>>> is clean already, which seems like it could be nice. But that seems
>>> weird modularity wise.
>>
>> Good point, we can block on the read-ahead. Which is counter intuitive,
>> but true.
> 
>> I'll queue up the below for now, better safe than sorry.
>>
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index fb5c5b3e23f4..1464e4c9b04c 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -2728,8 +2728,7 @@ static int io_fadvise(struct io_kiocb *req, struct io_kiocb **nxt,
>>  	struct io_fadvise *fa = &req->fadvise;
>>  	int ret;
>>  
>> -	/* DONTNEED may block, others _should_ not */
>> -	if (fa->advice == POSIX_FADV_DONTNEED && force_nonblock)
>> +	if (force_nonblock)
>>  		return -EAGAIN;
>>  
>>  	ret = vfs_fadvise(req->file, fa->offset, fa->len, fa->advice);
> 
> Hm, that seems a bit broad. It seems fairly safe to leave
> POSIX_FADV_{NORMAL,RANDOM,SEQUENTIAL} as sync. I guess there's there's
> the argument that that's not something one does frequently enough to
> care, but it's not hard to imagine wanting to change to RANDOM for a few
> reads and then back to NORMAL.

Yeah agree, not sure why I didn't cater to the normal cases. I'll
adjust.

-- 
Jens Axboe