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Date: Mon, 06 Dec 2004 17:04:38 +0100
From: =?ISO-8859-1?Q?Jean-S=E9bastien_P=E9dron?= <jspedron@club-internet.fr>
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To: Doug Barton <DougB@freebsd.org>
References: <20041204163103.M787@cp254-91.vpnaapncrgbja.pb.mn>
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cc: Poul-Henning Kamp <phk@phk.freebsd.dk>
cc: freebsd-current@freebsd.org
Subject: Re: panic: Inconsistent vnode bufstrategy
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Doug Barton wrote:
> I'm using mount_ntfs to read data off my winxp partition on a dual boot
> system. For a week or so I've been getting a panic, which I've finally
> been able to get a dump for:
>
> [stack trace]
I get exactly the same panic() with NTFS and ReiserFS.
I think it's related with phk's commit about:
"Make VOP_BMAP return a struct bufobj for the underlying storage device
instead of a vnode for it." (dated Mon Nov 15 09:18:26 2004 UTC)
The panic() occurs with mmap(2), for example while cp(1)'ing files
smaller than 8 MB.
In file systems like ufs, msdosfs or ext2fs, VOP_BMAP will do:
*ap->a_bop = &VTOI(ap->a_vp)->i_devvp->v_bufobj;
So the vm will call the device bo_strategy callback, and everything works.
In NTFS, VOP_BMAP is different:
*ap->a_bop = &ap->a_vp->v_bufobj;
The goal is that the vm should call the file system's VOP_STRATEGY, not
the device directly (that's what I guess, I'm not familiar with this fs :-).
Here is what I understand in the code, when the vm read a page:
1) vnode_pager_generic_getpages prepare a new struct buf *bp
(sys/vm/vnode_pager.c:817) and, twice, will KASSERT() that bp->b_vp is
NULL (in initpbuf, called by getpbuf, and in pbgetbo).
2) in pbgetbo, bp->b_bufobj = bo (bo is "returned" by file system's
VOP_BMAP; see the "*ap->a_bop = ..." lines above)
2) vnode_pager_generic_getpages call bstrategy, which in turn call
bp->b_bufobj->bo_ops->bop_strategy(). For NTFS, the callback is
bufstrategy (sys/kern/vfs_bio.c:3826)
3) in bufstrategy, KASSERT(bp->b_vp == bo->bo_private, ("Inconsistent
vnode bufstrategy")); but bp->b_vp is NULL.
In ReiserFS, because small files are not 512b-aligned on the device and
the vm must call the fs' VOP_STRATEGY, I did the same thing in VOP_BMAP,
which triggered this panic().
Cc'd to phk, he surely know the internals better than me :-)
Jean-Seb
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