linux-kernel - Re: m(un)map kmalloc buffers to userspace

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Date:	Wed, 9 Dec 2015 16:12:55 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Sebastian Frias <sebastian_frias@...madesigns.com>
Cc:	Marc Gonzalez <marc_gonzalez@...madesigns.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: m(un)map kmalloc buffers to userspace

On Wed 09-12-15 15:53:22, Sebastian Frias wrote:
[...]
> 2) Now that VM_RESERVED was removed, is there another recommended flag to
> replace it for the purposes above?

VM_IO + potentially others depending on your usecase.

> 3) Since it was working before, we suppose that something that was
> previously done by default on the kernel it is not done anymore, could that
> be a remap_pfn_range during mmap or kmalloc?

VM_RESERVED removal was a cleanup which has removed the flag because it
was not needed and the same effect could be implied from either VM_IO or 
VM_DONTEXPAND | VM_DONTDUMP. See 314e51b9851b ("mm: kill vma flag
VM_RESERVED and mm->reserved_vm counter") for more detailed information.

> 4) We tried using remap_pfn_range inside mmap and while it seems to work, we
> still get occasional crashes due to corrupted memory (in this case the
> behaviour is the same between 4.1 and 3.4 when using the same modified
> driver), are we missing something?

This is hard to tell without knowing your driver. I would just encourage
you to look at other drivers which map kernel memory to userspace via
mmap. There are many of them. Maybe you can find a pattern which suites
your usecase.

-- 
Michal Hocko
SUSE Labs
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