Re: locking user space memory in kernel

[Ross Dickson]

Libor Michalek wrote: 
 
>----- Forwarded message from Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> ----- 
 > 
 > 
 >>Date: Sun, 21 Mar 2004 12:31:59 +0100 
 >>From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> 
 >>To: Eli Cohen <mlxk@xxxxxxxxxxxxxx> 
 >>Cc: linux-kernel@xxxxxxxxxxxxxxx 
 >>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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