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Message-ID: <20230123164218.qaqqg3ggbymtlwjx@quack3>
Date: Mon, 23 Jan 2023 17:42:18 +0100
From: Jan Kara <jack@...e.cz>
To: Matthew Wilcox <willy@...radead.org>
Cc: David Howells <dhowells@...hat.com>,
John Hubbard <jhubbard@...dia.com>,
Al Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@...radead.org>,
Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
Jeff Layton <jlayton@...nel.org>,
Logan Gunthorpe <logang@...tatee.com>,
linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or
just list)
On Mon 23-01-23 16:31:32, Matthew Wilcox wrote:
> On Fri, Jan 20, 2023 at 05:55:48PM +0000, David Howells wrote:
> > (3) Make the bio struct carry a pair of flags to indicate the cleanup
> > mode. BIO_NO_PAGE_REF is replaced with BIO_PAGE_REFFED (equivalent to
> > FOLL_GET) and BIO_PAGE_PINNED (equivalent to BIO_PAGE_PINNED) is
> > added.
>
> I think there's a simpler solution than all of this.
>
> As I understand the fundamental problem here, the question is
> when to copy a page on fork. We have the optimisation of COW, but
> O_DIRECT/RDMA/... breaks it. So all this page pinning is to indicate
> to the fork code "You can't do COW to this page".
>
> Why do we want to track that information on a per-page basis? Wouldn't it
> be easier to have a VM_NOCOW flag in vma->vm_flags? Set it the first
> time somebody does an O_DIRECT read or RDMA pin. That's it. Pages in
> that VMA will now never be COWed, regardless of their refcount/mapcount.
> And the whole "did we pin or get this page" problem goes away. Along
> with folio->pincount.
Well, but anon COW code is not the only (planned) consumer of the pincount.
Filesystems also need to know whether a (shared pagecache) page is pinned
and can thus be modified behind their backs. And for that VMA tracking
isn't really an option.
Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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