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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 9 Sep 2021 14:56:18 -0700
Message-ID: <CAHk-=whoMLW-WP=8DikhfE4xAu_Tw9jDNkdab4RGEWWMagzW8Q@mail.gmail.com>
Subject: Re: [git pull] iov_iter fixes
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Thu, Sep 9, 2021 at 2:39 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> OK, one that I immediately found is just doing O_DIRECT to a block
> device or file on XFS. As pages are mapped and added, the iov_iter is
> advanced. If we then go and submit and get -EAGAIN, for example, then we
> return with what we mapped already consumed.

Ok, that's annoying but understandable. Dave points to a commit that
removes one of the EAGAIN cases, but apparently not some others.

I do kind of wonder if you can't have the exact same case when *some*
of the IO succeeds, though.

IOW, can't we have  that

        ret = io_iter_do_read(req, iter);

return partial success - and if XFS does that "update iovec on
failure", I could easily see that same code - or something else -
having done the exact same thing.

Put another way: if the iovec isn't guaranteed to be coherent when an
actual error occurs, then why would it be guaranteed to be coherent
with a partial success value?

Because in most cases - I'd argue pretty much all - those "partial
success" cases are *exactly* the same as the error cases, it's just
that we had a loop and one or more iterations succeeded before it hit
the error case.

Hmm?

            Linus