linux-kernel - Re: [git pull] iov

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Message-ID: <565e8aa7-27e7-4c3c-1a84-4181194c74d8@kernel.dk>
Date:   Fri, 10 Sep 2021 09:36:06 -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 9:32 AM, Al Viro wrote:
> On Fri, Sep 10, 2021 at 09:08:02AM -0600, Jens Axboe wrote:
> 
>>> You actually can cut it down even more - nr_segs + iov remains constant
>>> all along, so you could get away with just 3 words here...  I would be
>>
>> Mmm, the iov pointer remains constant? Maybe I'm missing your point, but
>> the various advance functions are quite happy to increment iter->iov or
>> iter->bvec, so we need to restore them. From a quick look, looks like
>> iter->nr_segs is modified for advancing too.
>>
>> What am I missing?
> 
> i->iov + i->nr_segs does not change - the places incrementing the former
> will decrement the latter by the same amount.  So it's enough to store
> either of those - the other one can be recovered by subtracting the
> saved value from the current i->iov + i->nr_segs.

Ahh, clever. Yes that should work just fine. Let me test that and send
out a proposal. Thanks Al.

-- 
Jens Axboe