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Message-ID: <324815.1674494391@warthog.procyon.org.uk>
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
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