linux-kernel - Re: [PATCH v2 7/7] virtio_balloon: Stop calling page_address() in free

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Message-ID: <20250830074747-mutt-send-email-mst@kernel.org>
Date: Sat, 30 Aug 2025 07:48:20 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
	David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Moore <paul@...l-moore.com>,
	Stephen Smalley <stephen.smalley.work@...il.com>,
	Ondrej Mosnacek <omosnace@...hat.com>, selinux@...r.kernel.org
Subject: Re: [PATCH v2 7/7] virtio_balloon: Stop calling page_address() in
 free_pages()

On Thu, Aug 28, 2025 at 03:53:56PM +0100, Matthew Wilcox wrote:
> On Wed, Aug 27, 2025 at 11:29:22AM -0700, Vishal Moola (Oracle) wrote:
> > I imagine theres more of these lingering in the kernel, but theres so
> > many callers and I only looked for the ones that were calling
> > page_address() inline :(.
> 
> There's only 841 callers of free_page() and free_pages()!
> 
> It's a bit of a disease we have, to be honest,  Almost all of
> them should be using kmalloc() instead.  To pick on one at random,
> sel_read_bool() in security/selinux/selinuxfs.c is the implementation
> of read() for some file in selinux.  All it's trying to do is output two
> numbers, so it allocates an entire page of memory, prints two numbers
> to it (while being VERY CAREFUL not to overflow the buffer!) and copies
> the buffer to userspace.
> 
> It should just use kmalloc.

Why even kmalloc? Why not have a small array on stack?

>  Oh, and it should avoid leaking the buffer
> if security_get_bool_value() returns an error.