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SubjectRe: locking user space memory in kernel
Date
Libor Michalek wrote: 

>----- Forwarded message from Manfred Spraul <manfred@colorfullife.com> -----
>
>
>>Date: Sun, 21 Mar 2004 12:31:59 +0100
>>From: Manfred Spraul <manfred@colorfullife.com>
>>To: Eli Cohen <mlxk@mellanox.co.il>
>>Cc: linux-kernel@vger.kernel.org
>>Subject: Re: locking user space memory in kernel
>>
>>Hi Eli,
>>
>>I think just get_user_pages() should be sufficient: the pages won't be
>>swapped out. You don't need to set VM_LOCKED in vma->vm_flags to prevent
>>the swap out. In the worst case, the pte is cleared a that will cause a
>>soft page fault, but the physical address won't change. Multiple
>>get_user_pages() calls on overlapping regions are ok, the page count is
>>an atomic_t, at least 24-bit large.
>>
>>
>
> The soft page fault is a problem if the device is going to write data
>into the buffer and then notify the user that the buffer now contains
>valid data. If the soft page fault occurs before the device has written
>to the page list, once the user is notified of the write and reads the
>buffer, it will no longer be the same pages as the ones to which the
>device wrote.
>

I know of an open source driver for image acquisition cards iti-fg that does
"PAGEWISE transfer of image frames directly into user space". The
release notes mention the memory locking.

It is available here
http://oss.gom.com/

Regards
Ross.

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