Subject: Re: M_READONLY
To: YAMAMOTO Takashi <>
From: Jason Thorpe <>
List: tech-net
Date: 09/20/2004 14:52:21
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On Sep 20, 2004, at 12:57 PM, YAMAMOTO Takashi wrote:

> i think that checking M_EXT_ROMAP is enough for loaned buffers
> because they're read-only mapped in kernel.

For loaned buffers, I agree.  But I'm talking about buffers that might 
not necessarily be loaned in the traditional sense.

Doesn't the NFS code attach the contents of a struct buf to an mbuf to 
achieve zero-copy sends?  The contents of that struct buf should really 
not be modified, but your patch would make the mbuf code assume that it 
can be.

> if it isn't enough (although there's currently no case i can think of),
> we can introduce a new flag, say, M_EXT_READONLY.

I would say that the existing behavior should remain, and non-cluster 
external storage that is R/W'able should explicitly set an M_EXT_RW 

         -- Jason R. Thorpe <>

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