linux-kernel - Could it be made possible to offer "supplementary" data to a DIO write ?

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Date:   Thu, 05 Aug 2021 11:19:17 +0100
From:   David Howells <dhowells@...hat.com>
To:     linux-fsdevel@...r.kernel.org
cc:     dhowells@...hat.com, jlayton@...nel.org,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        dchinner@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Could it be made possible to offer "supplementary" data to a DIO write ?

Hi,

I'm working on network filesystem write helpers to go with the read helpers,
and I see situations where I want to write a few bytes to the cache, but have
more available that could be written also if it would allow the
filesystem/blockdev to optimise its layout.

Say, for example, I need to write a 3-byte change from a page, where that page
is part of a 256K sequence in the pagecache.  Currently, I have to round the
3-bytes out to DIO size/alignment, but I could say to the API, for example,
"here's a 256K iterator - I need bytes 225-227 written, but you can write more
if you want to"?

Would it be useful/feasible to have some sort of interface that allows the
offer to be made?

David