Date: Wed, 13 Nov 2002 15:24:42 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: Daniel O'Connor <doconnor@gsoft.com.au>, Hans Zaunere <zaunere@yahoo.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: Shared files within a jail Message-ID: <3DD2DF3A.18489E80@mindspring.com> References: <20021113034726.75787.qmail@web12801.mail.yahoo.com> <1037159767.66058.34.camel@chowder.localdomain> <200211130530.gAD5UxNt067928@apollo.backplane.com> <3DD1FAB9.82607C41@mindspring.com> <200211131114.gADBE3lM069566@apollo.backplane.com>
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Matthew Dillon wrote: > :> Try using null mounts. The warning is in there because making the > :> null mount code work is a real hack and the authors aren't entirely > :> sure that everything's gotten covered. That said, use of a null mount > :> is certainly a lot safer if the stuff behind the mount is mostly > :> static. > : > :The problem is in the VM object alias code. Specifically, the > :getpages/putpages have to be implemented in terms of read/write, > :so that there are not two vm_object_t's that refer to the same > :data, since there is no "upcall" to notify of changes in a lower > :layer, and therefore guarantee coherency. > > I'm fairly sure the VM issues were fixed when VOP_GETVOBJECT was > added. A file accessed via a null mount will have the same VM object > as the file in the original filesystem. I'm not 100% sure about > that, I wasn't the one who did it, but I seem to recall it being > discussed. VOP_GETVOBJECT is a different name, but the VOP was my suggestion, to allow an upper layer to obtain a backing object, and to collapse intermediate layers. The issue is that the NULLFS getpages falls through the the vfs_default.c vop_stdgetpages(), which calls the function vnode_pager_generic_getpages(), which in turn, calls VOP_BMAP(), which in null_vnops.c is vop_eopnotsupp(), so it falls back to vnode_pager_input_smlfs(), which VOP_BMAP()'s *again*, but off the device. At which point, you've lost coherency. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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