Re: [PATCH v2 0/4] open/accept directly into io_uring fixed file table

[Pavel Begunkov]

On 8/13/21 8:00 PM, Josh Triplett wrote:
> On Fri, Aug 13, 2021 at 05:43:09PM +0100, Pavel Begunkov wrote:
>> Add an optional feature to open/accept directly into io_uring's fixed
>> file table bypassing the normal file table. Same behaviour if as the
>> snippet below, but in one operation:
>>
>> sqe = prep_[open,accept](...);
>> cqe = submit_and_wait(sqe);
>> // error handling
>> io_uring_register_files_update(uring_idx, (fd = cqe->res));
>> // optionally
>> close((fd = cqe->res));
>>
>> The idea in pretty old, and was brough up and implemented a year ago
>> by Josh Triplett, though haven't sought the light for some reasons.
> 
> Thank you for working to get this over the finish line!
> 
>> Tested on basic cases, will be sent out as liburing patches later.
>>
>> A copy paste from 2/2 describing user API and some notes:
>>
>> The behaviour is controlled by setting sqe->file_index, where 0 implies
>> the old behaviour. If non-zero value is specified, then it will behave
>> as described and place the file into a fixed file slot
>> sqe->file_index - 1. A file table should be already created, the slot
>> should be valid and empty, otherwise the operation will fail.
>>
>> Note 1: we can't use IOSQE_FIXED_FILE to switch between modes, because
>> accept takes a file, and it already uses the flag with a different
>> meaning.
>>
>> Note 2: it's u16, where in theory the limit for fixed file tables might
>> get increased in the future. If would ever happen so, we'll better
>> workaround later, e.g. by making ioprio to represent upper bits 16 bits.
>> The layout for open is tight already enough.
> 
> Rather than using sqe->file_index - 1, which feels like an error-prone
> interface, I think it makes sense to use a dedicated flag for this, like
> IOSQE_OPEN_FIXED. That flag could work for any open-like operation,
> including open, accept, and in the future many other operations such as
> memfd_create. (Imagine using a single ring submission to open a memfd,
> write a buffer into it, seal it, send it over a UNIX socket, and then
> close it.)
> 
> The only downside is that you'll need to reject that flag in all
> non-open operations. One way to unify that code might be to add a flag
> in io_op_def for open-like operations, and then check in common code for
> the case of non-open-like operations passing IOSQE_OPEN_FIXED.

io_uring is really thin, and so I absolutely don't want any extra
overhead in the generic path, IOW anything affecting
reads/writes/sends/recvs.

The other reason is that there are only 2 bits left in sqe->flags,
and we may use them for something better, considering that it's
only open/accept and not much as this.

I agree that it feels error-prone, but at least it can be wrapped
nicely enough in liburing, e.g.

void io_uring_prep_openat_direct(struct io_uring_sqe *sqe, int dfd,
				 const char *path, int flags,
				 mode_t mode, int slot_idx);


> Also, rather than using a 16-bit index for the fixed file table and
> potentially requiring expansion into a different field in the future,
> what about overlapping it with the nofile field in the open and accept
> requests? If they're not opening a normal file descriptor, they don't
> need nofile. And in the original sqe, you can then overlap it with a
> 32-bit field like splice_fd_in.

There is no nofile in SQEs, though

req->open.nofile = rlimit(RLIMIT_NOFILE);
 
> EEXIST seems like the wrong error-code to use if the index is already in
> use; open can already return EEXIST if you pass O_EXCL. How about EBADF,
> or better yet EBADSLT which is unlikely to be returned for any other
> reason?

Sure, sounds better indeed!

-- 
Pavel Begunkov