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To: Matthew Dillon <dillon@apollo.backplane.com>
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Date: Wed, 21 Apr 2004 21:52:43 -0400
From: Stephan Uphoff <ups@tree.com>
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cc: Julian Elischer <julian@elischer.org>
cc: Stephan Uphoff <ups@tree.com>
Subject: Re: how to flush out cache.? 
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Matthew Dillon wrote:
> :
> :> 
> :> mmap() and msync(..MS_INVALIDATE..) should work.
> :
> :hmmm that is rather interesting..
> :I wonder if it would work....
> :Maybe a vm guru could confirm this.. (under 4.x)
> :
> 
>     Huh.  If I hadn't looked at the code I would have said that
>     MS_INVALIDATE doesn't work in FreeBSD, but when I look at the code
>     it sure looks like it ought to work!
> 
>     But, alas, it does not.  The invalidation request goes all the way
>     through to the vnode pager but it looks like the vnode pager ignores
>     it.
> 
>     MS_INVALIDATE -> OBJPC_INVAL -> VM_PAGER_PUT_INVAL -> IO_INVAL -> (ignored)
> 
>     IO_INVAL is defined to be 'invalidate after I/O completes', 
>     not 'throw away the dirty data', but the only place it appears to be 
>     implemented is in the NFS code.
> 
> 						-Matt


Yes - but FreeBSD then calls vm_object_page_remove to remove the pages 
from the vnode object. (vm_object_sync for 5.x or vm_map_clean for 4.x )

Mhhh .. 4.X  removes the pages without flushing if the file is not write 
mapped.
This means that anyone with read access can screw up a file.
( For example periodically throwing away pages from /var/log/wtmp ...)
5.x looks better - but there is a race condition that can cause the same thing.
I think for msync() the vm_object_page_remove() function should be called with 
clean_only set to TRUE to avoid throwing away valid changes.
( I will send-pr tomorrow )

	Stephan