linux-kernel - Re: [git pull] iov

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Date:   Fri, 10 Sep 2021 11:04:02 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] iov_iter fixes

On 9/10/21 10:56 AM, Al Viro wrote:
> On Fri, Sep 10, 2021 at 10:06:25AM -0600, Jens Axboe wrote:
> 
>> Looks something like this. Not super pretty in terms of needing a define
>> for this, and maybe I'm missing something, but ideally we'd want it as
>> an anonymous struct that's defined inside iov_iter. Anyway, gets the
>> point across. Alternatively, since we're down to just a few members now,
>> we just duplicate them in each struct...
>>
>> Would be split into two patches, one for the iov_state addition and
>> the save/restore helpers, and then one switching io_uring to use them.
>> Figured we'd need some agreement on this first...
> 
>> +#define IOV_ITER_STATE					\
>> +	size_t iov_offset;				\
>> +	size_t count;					\
>> +	union {						\
>> +		unsigned long nr_segs;			\
>> +		struct {				\
>> +			unsigned int head;		\
>> +			unsigned int start_head;	\
>> +		};					\
>> +		loff_t xarray_start;			\
>> +	};						\
>> +
>> +struct iov_iter_state {
>> +	IOV_ITER_STATE;
>> +};
>> +
>>  struct iov_iter {
>>  	u8 iter_type;
>>  	bool data_source;
>> -	size_t iov_offset;
>> -	size_t count;
>>  	union {
>>  		const struct iovec *iov;
>>  		const struct kvec *kvec;
>> @@ -40,12 +54,10 @@ struct iov_iter {
>>  		struct pipe_inode_info *pipe;
>>  	};
>>  	union {
>> -		unsigned long nr_segs;
>> +		struct iov_iter_state state;
>>  		struct {
>> -			unsigned int head;
>> -			unsigned int start_head;
>> +			IOV_ITER_STATE;
>>  		};
>> -		loff_t xarray_start;
>>  	};
>>  	size_t truncated;
>>  };
> 
> No.  This is impossible to read *and* wrong for flavours other than
> iovec anyway.
> 
> Rules:
> 	count is flavour-independent
> 	iovec: iov, nr_segs, iov_offset.  nr_segs + iov is constant
> 	kvec: kvec, nr_segs, iov_offset.  nr_segs + kvec is constant
> 	bvec: bvec, nr_segs, iov_offset.  nr_segs + bvec is constant
> 	xarray: xarray, xarray_start, iov_offset.  xarray and xarray_start are constant.
> 	pipe: pipe, head, start_head, iov_offset.  pipe and start_head are constant,
> 						   iov_offset can be derived from the rest.
> 	discard: nothing.
> 
> What's more, for pipe (output-only) the situation is much trickier and
> there this "reset + advance" won't work at all.  Simply not applicable.
> 
> What's the point of all those contortions, anyway?  You only need it for
> iovec case; don't mix doing that and turning it into flavour-independent
> primitive.

Yes that's a good point, BVEC as well fwiw. But those two are very
similar.

> Especially since you turn around and access the fields of that sucker
> (->count, that is) directly in your code.  Keep it simple and readable,
> please.  We'll sort the sane flavour-independent API later.  And get
> rid of ->truncate, while we are at it.

Alright, so how about I just make the state a bit dumber and only work
for iovec/bvec. That gets rid of the weirdo macro. Add a WARN_ON_ONCE()
for using restore on anything that isn't an IOVEC/BVEC.

Sound reasonable?

-- 
Jens Axboe