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

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Message-ID: <1216854.1628177932@warthog.procyon.org.uk>
Date:   Thu, 05 Aug 2021 16:38:52 +0100
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
        jlayton@...nel.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: Re: Could it be made possible to offer "supplementary" data to a DIO write ?

Matthew Wilcox <willy@...radead.org> wrote:

> > Note that PAGE_SIZE varies across arches and folios are going to
> > exacerbate this.  What I don't want to happen is that you read from a
> > file, it creates, say, a 4M (or larger) folio; you change three bytes and
> > then you're forced to write back the entire 4M folio.
> 
> Actually, you do.  Two situations:
> 
> 1. Application uses MADVISE_HUGEPAGE.  In response, we create a 2MB
> page and mmap it aligned.  We use a PMD sized TLB entry and then the
> CPU dirties a few bytes with a store.  There's no sub-TLB-entry tracking
> of dirtiness.  It's just the whole 2MB.

That's a special case.  The app specifically asked for it.  I'll grant with
mmap you have to mark a whole page as being dirty - but if you mmapped it, you
need to understand that's what will happen.

> 2. The bigger the folio, the more writes it will absorb before being
> written back.  So when you're writing back that 4MB folio, you're not
> just servicing this 3 byte write, you're servicing every other write
> which hit this 4MB chunk of the file.

You can argue it that way - but we already do it bytewise in some filesystems,
so what you want would necessitate a change of behaviour.

Note also that if the page size > max RPC payload size (1MB in NFS, I think),
you have to make multiple write operations to fulfil that writeback; further,
if you have an object-based system you might be making writes to multiple
servers, some of which will not actually make a change, to make that
writeback.

I wonder if this needs pushing onto the various network filesystem mailing
lists to find out what they want and why.

David