Re: Propagating GFP_NOFS inside __vmalloc()

["Ricardo M. Correia" <ricardo.correia@oracle.com>]

On Thu, 2010-11-11 at 14:25 -0800, Andrew Morton wrote:
> And then we can set current->gfp_mask to GFP_ATOMIC when we take an
> interrupt, or take a spinlock.
> 
> And leave it at GFP_KERNEL when in process context.
> 
> And switch GFP_KERNEL to GFP_NOFS in the VM.
> 
> And switch to GFP_NOIO in the block layer.
> 
> So the allocation mode becomes implicit to the task state, so callers
> usually don't need to track it.
> 
> So, ultimately, kmalloc(), alloc_pages() etc don't actually need a mode
> arg at all.  We'll need new, special functions which _do_ take the
> gfp_t but they will be rarely-called specialised things.
>
> And probably we'll need interfaces like
> 
> 	gfp_t mm_set_alloc_mode(gfp_t flags);
> 	void mm_restore_alloc_mode(gfp_t flags);
> 
> 	gfp_t flags;
> 
> 	flags = mm_set_alloc_mode(GFP_NOIO);
> 	...
> 	mm_restore_alloc_mode(flags);

Actually, I think it may not be that simple...

Looking at some of the __GFP_* flags, it seems that some of them look
like allocation "options", i.e. something we may want or may not want to
do on a certain allocation, others look more like "capabilities", i.e.
something that we can or cannot do in a certain context.

For example, __GFP_ZERO, __GFP_REPEAT, __GFP_HIGHMEM, ... is something
that we'd probably want a caller to specify on each allocation, because
only he knows what he actually wants to do.

Others, like __GFP_FS, __GFP_IO, __GFP_WAIT, are things that we either
can or cannot do, depending on the context that we're in.

The latter ones seem worth to start tracking on the task_struct, but the
former ones I think we'd still want to pass them to kmalloc() on each
invocation.

Fortunately, if we put the latter ones in the task_struct, it removes
the need for having to propagate gfp_flags from function to function.

And contrary to what you said previously (which at the time sounded
correct to me), this can actually save a lot of stack space, especially
on more register-starved architectures, because the only places where we
need to save the flags on the stack is when we enter/exit a certain
context, as opposed to having to always having to pass the gfp_mask down
the call stack like we do now.

> argh, someone save us.

:-)

Thanks,
Ricardo


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