Messages in this thread | | | Date | Tue, 11 Dec 2012 19:07:00 +0400 | From | Stanislav Kinsbursky <> | Subject | Re: [Devel] [PATCH 2/6] nfsd: swap fs root in NFSd kthreads |
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11.12.2012 18:56, J. Bruce Fields пишет: > On Tue, Dec 11, 2012 at 06:12:40PM +0400, Stanislav Kinsbursky wrote: >> UID: 9899 >> >> 11.12.2012 18:00, Stanislav Kinsbursky пишет: >>> 11.12.2012 00:28, J. Bruce Fields пишет: >>>> On Thu, Dec 06, 2012 at 06:34:47PM +0300, Stanislav Kinsbursky wrote: >>>>> NFSd does lookup. Lookup is done starting from current->fs->root. >>>>> NFSd is a kthread, cloned by kthreadd, and thus have global (but luckely >>>>> unshared) root. >>>>> So we have to swap root to those, which process, started NFSd, has. Because >>>>> that process can be in a container with it's own root. >>>> >>>> This doesn't sound right to me. >>>> >>>> Which lookups exactly do you see being done relative to >>>> current->fs->root ? >>>> >>> >>> Ok, you are right. I was mistaken here. >>> This is not a exactly lookup, but d_path() problem in svc_export_request(). >>> I.e. without root swapping, d_path() will give not local export path (like "/export") >>> but something like this "/root/containers_root/export". >>> >> >> We, actually, can do it less in less aggressive way. >> I.e. instead root swap and current svc_export_request() implementation: >> >> void svc_export_request(...) >> { >> <snip> >> pth = d_path(&exp->ex_path, *bpp, *blen); >> <snip> >> } >> >> we can do something like this: >> >> void svc_export_request(...) >> { >> struct nfsd_net *nn = ... >> <snip> >> spin_lock(&dcache_lock); >> pth = __d_path(&exp->ex_path, &nn->root, *bpp, *blen); >> spin_unlock(&dcache_lock); >> <snip> >> } > > That looks simpler, but I still don't understand why we need it. > > I'm confused about how d_path works; I would have thought that > filesystem namespaces would have their own vfsmount trees and hence that > the (vfsmount, dentry) would be enough to specify the path. Is the root > argument for the case of chroot? Do we care about that? >
It works very simple: just traverse the tree from specified dentry up to current->fs->root.dentry. Having container in some fully separated mount point is great, of course. But: 1) this is a limitation we really want to avoid. I.e. container can be chrooted into some path like "/root/containers_root/" as in example above. 2) NFSd kthread works in init root environment. But we anyway want to get proper path string in container root, but not in kthreads root.
> Also, svc_export_request is called from mountd's read of > /proc/net/rpc/nfsd.export/channel. If mountd's root is wrong, then > nothing's going to work anyway. >
I don't really understand, how mountd's root can be wrong. I.e. its' always right as I see it. NFSd kthreads have to swap/use relative path/whatever to communicate with proper mountd. Or I'm missing something?
> --b. >
-- Best regards, Stanislav Kinsbursky -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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