Re: CVS commit: src

[Maxime Villard <max%m00nbsd.net@localhost>]

Le 23/02/2020 à 23:19, Andrew Doran a écrit :
> On Fri, Feb 21, 2020 at 02:14:31PM +0100, Kamil Rytarowski wrote:
>
> > On 22.12.2019 20:47, Andrew Doran wrote:
> > > Module Name:	src
> > > Committed By:	ad
> > > Date:		Sun Dec 22 19:47:35 UTC 2019
> > >
> > > Modified Files:
> > > 	src/external/cddl/osnet/dist/uts/common/fs/zfs: zfs_ctldir.c
> > > 	src/sys/kern: vfs_mount.c vfs_subr.c vfs_syscalls.c
> > > 	src/sys/miscfs/genfs: genfs_vfsops.c
> > > 	src/sys/nfs: nfs_export.c
> > > 	src/sys/sys: mount.h vnode.h vnode_impl.h
> > > 	src/sys/ufs/lfs: ulfs_vfsops.c
> > > 	src/sys/ufs/ufs: ufs_vfsops.c ufs_wapbl.c
> > >
> > > Log Message:
> > > Make mntvnode_lock per-mount, and address false sharing of struct mount.
> >
> > This change broke kUBSan syzbot.
> >
> > The sanitizer is now very noisy as struct mount requires 64 byte alignment.
> >
> > http://netbsd.org/~kamil/kubsan/mount-alignment.txt
>
> I had a look this weekend.  This is down to KMEM_SIZE messing up the
> alignment, so is a DIAGNOSTIC thing.  The align_offset parameter to
> pool_cache() would be a nice easy way to solve this but it seems someone
> killed that off, so I'll need to give this some more thought.
>
> Andrew

kmem guarantees alignment on 8 bytes, but not more. Changing the backend
allocators to enforce more alignment still may not result in aligned buffers,
because kmem has the right to modify the buffers for debugging purposes, like
with KMEM_SIZE.

If you want a buffer aligned to a specific value, don't use kmem, rather a
pool(_cache) with COHERENCY_UNIT in "align".

If the goal is to have kmem really enforce COHERENCY_UNIT alignment, then this
should be documented and the debugging features should be adapted to respect
that constraint.

"align_offset" got removed because it increased complexity in subr_pool for
no reason (two users in all of the kernel, one was actually a bug).