From owner-freebsd-current  Mon Jan 18 06:17:49 1999
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	Mon, 18 Jan 1999 06:17:32 -0800 (PST)
Date: Mon, 18 Jan 1999 06:17:32 -0800 (PST)
From: Chris Timmons <skynyrd@opus.cts.cwu.edu>
To: Matthew Dillon <dillon@apollo.backplane.com>
cc: freebsd-current@FreeBSD.ORG, John Polstra <jdp@polstra.com>,
        luoqi@FreeBSD.ORG
Subject: Re: NFS problem found - pleaes try this patch.
In-Reply-To: <199901181336.FAA72182@apollo.backplane.com>
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Good work!  I have to run at the moment but it looks like you nailed this
one.  Your explanation coincides perfectly with the symptoms.

Thanks!

-Chris


On Mon, 18 Jan 1999, Matthew Dillon wrote:

>     Ok, I believe I have found the bug.  Please test the patch included below.
>     I was able to make /usr/ports/x11/XFree86-contrib after applying this 
>     patch ( and it was screwing up prior to that ).
> 
>     The problem is in getblk() - code was added to validate the buffer and
>     to clear B_CACHE if the bp was not entirely valid.  The problem is 
>     that NFS uses B_CACHE to flag a dirty buffer that needs to be written out!
>     Additionally, a write() to an NFS based file may write data that is not
>     on a DEV_BSIZE'd boundry which causes a subsequent read() to improperly
>     clear B_CACHE.
> 
>     There are almost certainly more problems like this -- using B_CACHE to
>     mark a buffer dirty is just plain dumb, it's no wonder NFS is so screwed 
>     up!
> 
> 					-Matt
> 
> 					Matthew Dillon 
> 					<dillon@backplane.com>
> 
> Index: kern/vfs_bio.c
> ===================================================================
> RCS file: /home/ncvs/src/sys/kern/vfs_bio.c,v
> retrieving revision 1.192
> diff -u -r1.192 vfs_bio.c
> --- vfs_bio.c	1999/01/12 11:59:34	1.192
> +++ vfs_bio.c	1999/01/18 13:25:27
> @@ -1364,6 +1364,7 @@
>  				break;
>  			}
>  		}
> +
>  		boffset = (i << PAGE_SHIFT) - (bp->b_offset & PAGE_MASK);
>  		if (boffset < bp->b_dirtyoff) {
>  			bp->b_dirtyoff = max(boffset, 0);
> @@ -1457,7 +1458,14 @@
>  		}
>  		KASSERT(bp->b_offset != NOOFFSET, 
>  		    ("getblk: no buffer offset"));
> +#if 0
>  		/*
> +		 * XXX REMOVED XXX - this is bogus.  It will cause the
> +		 * B_CACHE flag to be cleared for a partially constituted
> +		 * dirty buffer (NFS) that happens to have a write that is
> +		 * not on a DEV_BSIZE boundry!!!!!!  XXX REMOVED XXXX
> +		 */
> +		/*
>  		 * Check that the constituted buffer really deserves for the
>  		 * B_CACHE bit to be set.  B_VMIO type buffers might not
>  		 * contain fully valid pages.  Normal (old-style) buffers
> @@ -1478,6 +1486,7 @@
>  				poffset = 0;
>  			}
>  		}
> +#endif
>  
>  		if (bp->b_usecount < BUF_MAXUSE)
>  			++bp->b_usecount;
> 


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