Re: [PATCH 2/7] mm: introduce kvmalloc and kvmalloc_node

[David Rientjes <rientjes@google.com>]

On Wed, 15 Jul 2015, Dave Chinner wrote:

> > Sure, but it's not accomplishing the same thing: things like 
> > ext4_kvmalloc() only want to fallback to vmalloc() when high-order 
> > allocations fail: the function is used for different sizes.  This cannot 
> > be converted to kvmalloc_node() since it fallsback immediately when 
> > reclaim fails.  Same issue with single_file_open() for the seq_file code.  
> > We could go through every kmalloc() -> vmalloc() fallback for more 
> > examples in the code, but those two instances were the first I looked at 
> > and couldn't be converted to kvmalloc_node() without work.
> > 
> > > It is always easier to shoehorn utility functions locally within a
> > > subsystem (be it ext4, dm, etc) but once enough do something in a
> > > similar but different way it really should get elevated.
> > > 
> > 
> > I would argue that
> > 
> > void *ext4_kvmalloc(size_t size, gfp_t flags)
> > {
> > 	void *ret;
> > 
> > 	ret = kmalloc(size, flags | __GFP_NOWARN);
> > 	if (!ret)
> > 		ret = __vmalloc(size, flags, PAGE_KERNEL);
> > 	return ret;
> > }
> > 
> > is simple enough that we don't need to convert it to anything.
> 
> Except that it will have problems with GFP_NOFS context when the pte
> code inside vmalloc does a GFP_KERNEL allocation. Hence we have
> stuff in other subsystems (such as XFS) where we've noticed lockdep
> whining about this:
> 

Does anyone have an example of ext4_kvmalloc() having a lockdep violation?  
Presumably the GFP_NOFS calls to ext4_kvmalloc() will never have 
size > (1 << (PAGE_SHIFT + PAGE_ALLOC_COSTLY_ORDER)) so that kmalloc() 
above actually never returns NULL and __vmalloc() only gets used for the 
ext4_kvmalloc(..., GFP_KERNEL) call.

It should be fixed, though, probably in the same way as 
kmem_zalloc_large() today, but it seems the real fix would be to attack 
the whole vmalloc() GFP_KERNEL issue that has been talked about several 
times in the past.  Then the existing ext4_kvmalloc() implementation 
should be fine.

Once that's done, we can revisit the idea of a generalized kvmalloc() or 
kvmalloc_node(), but since the implementation such as above is different 
from the proposed kvmalloc_node() implementation with respect to 
high-order allocations, I doubt a generalized form will be helpful.

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