Re: [PATCH 3/4] mm: Gather more PFNs before sending a TLB to flush unmapped pages

Peter Zijlstra
2015-04-15T11:42:41Z

On Wed, Apr 15, 2015 at 11:42:55AM +0100, Mel Gorman wrote:
> +/*
> + * Use a page to store as many PFNs as possible for batch unmapping. Adjusting
> + * this trades memory usage for number of IPIs sent
> + */
> +#define BATCH_TLBFLUSH_SIZE \
> + ((PAGE_SIZE - sizeof(struct cpumask) - sizeof(unsigned long)) / sizeof(unsigned long))
>
> /* Track pages that require TLB flushes */
> struct unmap_batch {
> + /* Update BATCH_TLBFLUSH_SIZE when adjusting this structure */
> struct cpumask cpumask;
> unsigned long nr_pages;
> unsigned long pfns[BATCH_TLBFLUSH_SIZE];

The alternative is something like:

struct unmap_batch {
struct cpumask cpumask;
unsigned long nr_pages;
unsigned long pfnsp[0];
};

#define BATCH_TLBFLUSH_SIZE ((PAGE_SIZE - sizeof(struct unmap_batch)) / sizeof(unsigned long))

and unconditionally allocate 1 page. This saves you from having to worry
about the layout of struct unmap_batch.