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Date:   Tue, 22 Jun 2021 15:58:36 -0400
From:   'Alan Stern' <stern@rowland.harvard.edu>
To:     David Laight <David.Laight@aculab.com>
Cc:     'Mauro Carvalho Chehab' <mchehab+huawei@kernel.org>,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
        "linuxarm@huawei.com" <linuxarm@huawei.com>,
        "mauro.chehab@huawei.com" <mauro.chehab@huawei.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH v3] media: uvc: don't do DMA on stack
Message-ID: <20210622195836.GA468074@rowland.harvard.edu>
References: <6832dffafd54a6a95b287c4a1ef30250d6b9237a.1624282817.git.mchehab+huawei@kernel.org>
 <d33c39aa824044ad8cacc93234f1e1cd@AcuMS.aculab.com>
 <20210622132922.GB452785@rowland.harvard.edu>
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On Tue, Jun 22, 2021 at 02:21:27PM +0000, David Laight wrote:
> From: Alan Stern
> > Sent: 22 June 2021 14:29
> ...
> > > Thought...
> > >
> > > Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into
> > > a cache line that will not be accessed by any other code?
> > > (This is slightly weaker than requiring a cache-line aligned
> > > pointer - but very similar.)
> > 
> > As I understand it, on architectures that do not have cache-coherent
> > I/O, kmalloc is guaranteed to return a buffer that is
> > cacheline-aligned and whose length is a multiple of the cacheline
> > size.
> > 
> > Now, whether that buffer ends up being accessed by any other code
> > depends on what your driver does with the pointer it gets from
> > kmalloc.  :-)
> 
> Thanks for the clarification.
> 
> Most of the small allocates in the usb stack are for transmits
> where it is only necessary to ensure a cache write-back.
> 
> I know there has been some confusion because one of the
> allocators can add a small header to every allocation.
> This can lead to unexpectedly inadequately aligned pointers.
> If it is updated when the preceding block is freed (as some
> user-space mallocs do) then it would need to be in a
> completely separate cache line.

If you really want to find out what the true story is, you should ask 
on the linux-mm mailing list.  The rest of us are not experts on this 
stuff.

Alan Stern