linux-kernel - Trying to use SLUB in an odd way

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Date:	Mon, 24 May 2010 14:39:48 +1200
From:	Charles Manning <manningc2@...rix.gen.nz>
To:	linux-kernel@...r.kernel.org
Subject: Trying to use SLUB in an odd way

Hi

YAFFS uses an internal almost slub-like allocator and I've been looking at 
moving it to use SLUB as part of an attempt to mainline yaffs.

In yaffs I create a lot of tiny objects which are allocated in blocks, like 
slub, and then managed in a free list. Very slubbish so far, but mine is far 
less intelligent than slub.

The difference though is that I can keep the objects for each mount separate 
and can dump the whole lot on umount without individually freeing objects. I 
just deallocate my whole cache.

There are two problems that I encountered in  moving to slub:
1) I want to keep each mount point separate, but slub just hooks up with an 
existing cache of the same size. I managed to trick slub into keeping yaffs 
objects in their own cache by assigning a fake ctor. That stops the 
combination (well at present anyway - could easy change in the future like if 
ctor gets dropped). Like a VW Bug: ugly but it gets you there....

2) If I dump a cache with existing in-use objects then slub gets upset and 
dumps warnings. I don't like the idea of just ignoring warnings. I also don't 
want to manually tear down trees etc when the existing  "just dump it" 
approach is a lot faster. Pity there is no "trust me I know what I'm doing" 
flag.

Questions:
A) Is there a better way to use slub to do this or is it better to just 
continue with my manual allocator?

B) Is it worth adding flags to kmem_cache_create() to say:
 a) Don't combine this slub with others.
 b) "Trust me I know what I'm doing": Allow the cache to be dumped with 
objects still allocated. 

If (B) makes sense I'll put together a patch.

-- Charles

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