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

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Date:   Mon, 23 Jan 2023 17:19:51 +0000
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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)

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

> > Wouldn't that potentially make someone's entire malloc() heap entirely NOCOW
> > if they did a single DIO to/from it.
> 
> Yes.  Would that be an actual problem for any real application?

Without auditing all applications that do direct I/O writes, it's hard to
say - but a big database engine, Oracle for example, forking off a process,
say, could cause a massive slow down as fork suddenly has to copy a huge
amount of malloc'd data unnecessarily[*].

[*] I'm making wild assumptions about how Oracle's DB engine works.

> > 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.
> 
> I'm struggling to see the problem here.  If the child hasn't exec'd, the
> parent and child are still in the same security domain.  The parent
> could have modified the buffer before calling fork().

It could still inadvertently change the data its parent set to write out.  The
child *shouldn't* be able to change the parent's in-progress write.  The most
obvious problem would be in something that does DIO from a stack buffer, I
think.

David