Re: [PATCH] xfs: use kmem_cache_free() for kmem_cache objects

[Rustam Kovhaev <rkovhaev@gmail.com>]

On Thu, Sep 30, 2021 at 10:13:40AM +0200, Vlastimil Babka wrote:
> On 9/30/21 06:42, Dave Chinner wrote:
> > On Wed, Sep 29, 2021 at 02:23:47PM -0700, Rustam Kovhaev wrote:
> >> For kmalloc() allocations SLOB prepends the blocks with a 4-byte header,
> >> and it puts the size of the allocated blocks in that header.
> >> Blocks allocated with kmem_cache_alloc() allocations do not have that
> >> header.
> >> 
> >> SLOB explodes when you allocate memory with kmem_cache_alloc() and then
> >> try to free it with kfree() instead of kmem_cache_free().
> >> SLOB will assume that there is a header when there is none, read some
> >> garbage to size variable and corrupt the adjacent objects, which
> >> eventually leads to hang or panic.
> >> 
> >> Let's make XFS work with SLOB by using proper free function.
> >> 
> >> Fixes: 9749fee83f38 ("xfs: enable the xfs_defer mechanism to process extents to free")
> >> Signed-off-by: Rustam Kovhaev <rkovhaev@gmail.com>
> > 
> > IOWs, XFS has been broken on SLOB for over 5 years and nobody
> > anywhere has noticed.
> > 
> > And we've just had a discussion where the very best solution was to
> > use kfree() on kmem_cache_alloc() objects so we didn't ahve to spend
> > CPU doing global type table lookups or use an extra 8 bytes of
> > memory per object to track the slab cache just so we could call
> > kmem_cache_free() with the correct slab cache.
> > 
> > But, of course, SLOB doesn't allow this and I was really tempted to
> > solve that by adding a Kconfig "depends on SLAB|SLUB" option so that
> > we don't have to care about SLOB not working.
> > 
> > However, as it turns out that XFS on SLOB has already been broken
> > for so long, maybe we should just not care about SLOB code and
> > seriously consider just adding a specific dependency on SLAB|SLUB...
> 
> I think it's fair if something like XFS (not meant for tiny systems AFAIK?)
> excludes SLOB (meant for tiny systems). Clearly nobody tried to use these
> two together last 5 years anyway.

+1 for adding Kconfig option, it seems like some things are not meant to
be together.

> Maybe we could also just add the 4 bytes to all SLOB objects, declare
> kfree() is always fine and be done with it. Yes, it will make SLOB footprint
> somewhat less tiny, but even whan we added kmalloc power of two alignment
> guarantees, the impact on SLOB was negligible.

I'll send a patch to add a 4-byte header for kmem_cache_alloc()
allocations.

> > Thoughts?
> > 
> > Cheers,
> > 
> > Dave.
> > 
>