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Re: [PATCH 17/19] VFS: set PF_FSTRANS while namespace_sem is held.

To: NeilBrown <neilb@xxxxxxx>
Subject: Re: [PATCH 17/19] VFS: set PF_FSTRANS while namespace_sem is held.
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Wed, 16 Apr 2014 05:46:18 +0100
Cc: linux-mm@xxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140416040337.10604.86740.stgit@xxxxxxxxxxxxxx>
References: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx> <20140416040337.10604.86740.stgit@xxxxxxxxxxxxxx>
Sender: Al Viro <viro@xxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Apr 16, 2014 at 02:03:37PM +1000, NeilBrown wrote:
> namespace_sem can be taken while various i_mutex locks are held, so we
> need to avoid reclaim from blocking on an FS (particularly loop-back
> NFS).

I would really prefer to deal with that differently - by explicit change of
gfp_t arguments of allocators.

The thing is, namespace_sem is held *only* over allocations, and not a lot
of them, at that - only mnt_alloc_id(), mnt_alloc_group_id(), alloc_vfsmnt()
and new_mountpoint().  That is all that is allowed.

Again, actual work with filesystems (setup, shutdown, remount, pathname
resolution, etc.) is all done outside of namespace_sem; it's held only
for manipulations of fs/{namespace,pnode}.c data structures and the only
reason it isn't a spinlock is that we need to do some allocations.

So I'd rather slap GFP_NOFS on those few allocations...

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