Re: [RFC PATCH 07/13] kernel/fork: Split and export 'mm_alloc' and 'mm_init'

[Till Smejkal <till.smejkal@googlemail.com>]

On Tue, 14 Mar 2017, David Laight wrote:
> From: Linuxppc-dev Till Smejkal
> > Sent: 13 March 2017 22:14
> > The only way until now to create a new memory map was via the exported
> > function 'mm_alloc'. Unfortunately, this function not only allocates a new
> > memory map, but also completely initializes it. However, with the
> > introduction of first class virtual address spaces, some initialization
> > steps done in 'mm_alloc' are not applicable to the memory maps needed for
> > this feature and hence would lead to errors in the kernel code.
> > 
> > Instead of introducing a new function that can allocate and initialize
> > memory maps for first class virtual address spaces and potentially
> > duplicate some code, I decided to split the mm_alloc function as well as
> > the 'mm_init' function that it uses.
> > 
> > Now there are four functions exported instead of only one. The new
> > 'mm_alloc' function only allocates a new mm_struct and zeros it out. If one
> > want to have the old behavior of mm_alloc one can use the newly introduced
> > function 'mm_alloc_and_setup' which not only allocates a new mm_struct but
> > also fully initializes it.
> ...
> 
> That looks like bugs waiting to happen.
> You need unchanged code to fail to compile.

Thank you for this hint. I can give the new mm_alloc function a different name so
that code that uses the *old* mm_alloc function will fail to compile. I just reused
the old name when I wrote the code, because mm_alloc was only used in very few
locations in the kernel (2 times in the whole kernel source) which made identifying
and changing them very easy. I also don't think that there will be many users in the
kernel for mm_alloc in the future because it is a relatively low level data
structure. But if it is better to use a different name for the new function, I am
very happy to change this.

Till