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SubjectRe: [patch] remove redundant iov segment check
> I wonder if it wouldn't be better to make this change as part of a  
> larger change that moves towards an explicit iovec container struct
> rather than bare 'struct iov *' and 'nr_segs' arguments. The struct
> could have a flag that expressed whether the elements had been
> checked. A helper could be called by the upper and lower code paths
> which does the checking, marks the flag, and avoids checking again if
> the flag is set.
>
> We've wanted an explicit struct in the past to avoid the multiple
> walks of iovecs that various paths do for their own reasons. The
> iovec walk that is checking for length wrapping could also be
> building a bitmap of length alignment that O_DIRECT could be using to
> test 512B alignment without having to walk the iovec again.

I suspect it should be rather trivial to get this started. As a first
step we simply add a

struct iodesc {
int nr_segs;
struct iovec ioc[]
};
And then we can add fields where nessecary. First a full_length one
to avoid the loops to calculate thw whole I/O size, then flags for
the alignment check, etc..
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