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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 27 Aug 2021 15:35:06 -0700
Message-ID: <CAHk-=whCCyxkk+wfDZ5bQNX62MfdprBLpy_RwpSFhFziA2Oecg@mail.gmail.com>
Subject: Re: [PATCH v7 16/19] iomap: Add done_before argument to iomap_dio_rw
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Andreas Gruenbacher <agruenba@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, ocfs2-devel@oss.oracle.com
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On Fri, Aug 27, 2021 at 2:32 PM Darrick J. Wong <djwong@kernel.org> wrote:
>
> No, because you totally ignored the second question:
>
> If the directio operation succeeds even partially and the PARTIAL flag
> is set, won't that push the iov iter ahead by however many bytes
> completed?
>
> We already finished the IO for the first page, so the second attempt
> should pick up where it left off, i.e. the second page.

Darrick, I think you're missing the point.

It's the *return*value* that is the issue, not the iovec.

The iovec is updated as you say. But the return value from the async
part is - without Andreas' patch - only the async part of it.

With Andreas' patch, the async part will now return the full return
value, including the part that was done synchronously.

And the return value is returned from that async part, which somehow
thus needs to know what predated it.

Could that pre-existing part perhaps be saved somewhere else? Very
possibly. That 'struct iomap_dio' addition is kind of ugly. So maybe
what Andreas did could be done differently. But I think you guys are
arguing past each other.

           Linus