linux-kernel - Re: [PATCH 002/437] fs: add generic read/write iterator helpers

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Date: Mon, 15 Apr 2024 14:11:56 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH 002/437] fs: add generic read/write iterator helpers

On 4/15/24 1:55 PM, Al Viro wrote:
> On Thu, Apr 11, 2024 at 09:12:22AM -0600, Jens Axboe wrote:
> 
>> +/* generic read side helper for drivers converting to ->read_iter() */
>> +ssize_t vfs_read_iter(struct kiocb *iocb, struct iov_iter *to,
>> +		      ssize_t (*read)(struct file *, char __user *,
>> +				     size_t, loff_t *))
>> +{
>> +	return do_loop_readv(iocb->ki_filp, to, &iocb->ki_pos, 0, read);
>> +}
>> +EXPORT_SYMBOL(vfs_read_iter);
>> +
>> +/* generic write side helper for drivers converting to ->write_iter() */
>> +ssize_t vfs_write_iter(struct kiocb *iocb, struct iov_iter *from,
>> +		       ssize_t (*write)(struct file *, const char __user *,
>> +				       size_t, loff_t *))
>> +{
>> +	return do_loop_writev(iocb->ki_filp, from, &iocb->ki_pos, 0, write);
>> +}
>> +EXPORT_SYMBOL(vfs_write_iter);
> 
> Wait a minute; just what do you expect to happen if that ever gets called
> for ITER_BVEC or ITER_XARRAY?

do_loop_{readv,writev} need to look like the one io_uring had, which was
just an augmented version of the fs/ version. At least for handling
anything that is IOVEC/UBUF/BVEC. Outside of that, should not be
callable for eg ITER_XARRAY, who would do that? We should probably add a
check at the top of each just to vet the iter type and -EINVAL if it's
not one of the supported ones. With a WARN_ON_ONCE(), I suspect.

I'll be posting the first patches separately again, I've made some local
tweaks, with some drivers that can support it as-is. Just haven't gotten
around to doing this weeks iteration on it yet.

-- 
Jens Axboe