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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <Y862ZL5umO30Vu/D@casper.infradead.org>
References: <Y862ZL5umO30Vu/D@casper.infradead.org> <20230120175556.3556978-1-dhowells@redhat.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     dhowells@redhat.com, John Hubbard <jhubbard@nvidia.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list)
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Date:   Mon, 23 Jan 2023 16:38:47 +0000
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Matthew Wilcox <willy@infradead.org> wrote:

> 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.

Wouldn't that potentially make someone's entire malloc() heap entirely NOC=
OW
if they did a single DIO to/from it.

Also you only mention DIO read - but what about "start DIO write; fork(); =
touch
buffer" in the parent - now the write buffer belongs to the child and they=
 can
affect the parent's write.

David