linux-kernel - Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list)

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Message-ID: <Y862ZL5umO30Vu/D@casper.infradead.org>
Date:   Mon, 23 Jan 2023 16:31:32 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     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 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.