Re: [PATCH v3] mm: Export {__}get_nr_swap_pages()

Johannes Weiner
2015-12-17T19:46:07Z

On Thu, Dec 17, 2015 at 06:15:44PM +0000, Dave Gordon wrote:
> Some modules, like i915.ko, use swappable objects and may try to swap
> them out under memory pressure (via the shrinker). Before doing so,
> they want to check using get_nr_swap_pages() to see if any swap space
> is available as otherwise they will waste time purging the object from
> the device without recovering any memory for the system. This requires
> the kernel function get_nr_swap_pages() to be exported to the modules.
>
> The current implementation of this function is as a static inline
> inside the header file swap.h>; this doesn't work when compiled in
> a module, as the necessary global data is not visible. The original
> proposed solution was to export the kernel global variable to modules,
> but this was considered poor practice as it exposed more than necessary,
> and in an uncontrolled fashion. Another idea was to turn it into a real
> (non-inline) function; however this was considered to unnecessarily add
> overhead for users within the base kernel.
>
> Therefore, to avoid both objections, this patch leaves the base kernel
> implementation unchanged, but adds a separate (read-only) functional
> interface for callers in loadable kernel modules (LKMs). Which definition
> is visible to code depends on the compile-time symbol MODULE, defined
> by the Kbuild system when building an LKM.

I'm sorry, but this is beyond silly. 19 lines of code to fix a
non-existent problem? This lacks any sort of proportionality.

NAK