Subject: Re: map user memory in kernel
To: Eric Haszlakiewicz <>
From: Steven M. Bellovin <>
List: tech-kern
Date: 11/05/2005 11:49:48
In message <>, Eric Haszlakiewicz writes
>On Sat, Nov 05, 2005 at 11:15:16AM -0500, Steven M. Bellovin wrote:
>> Unless I badly misunderstand the situation, it doesn't matter.  The 
>> "address" is nothing of the sort.  Think of it as a hash value; any 
>> process that is sleeping on that hash value will be awakened, but they 
>> should all check to see that the situation they're really waiting for 
>> has occurred.  Otherwise, they sleep again.  
>> To put it more bluntly: the system would still work correctly if every 
>> process used 0 as its sleep "address".  It wouldn't be very efficient, 
>> which is why things aren't done that way, but it would still be correct.
>	It _does_ matter if you're using the physical address as the key
>to pass to sleep/wakeup.  It wouldn't work correctly if the page got
>paged back in to a different physical address.  Then you've called sleep
>with one value and might call wakeup with a different one.
That's correct, but only under certain circumstances: if the wakeup 
process is recalculating the physical address.  In many cases, the 
wakeup address is stored in a per-instance data structure.

		--Steven M. Bellovin,