On 2018-10-11 10:24 a.m., Logan Gunthorpe wrote:

On 2018-10-11 7:37 a.m., Christoph Hellwig wrote:

quoted

quoted

+/*
+ * Log2 of the upper bound of the size of a struct page. Used for sizing
+ * the vmemmap region only, does not affect actual memory footprint.
+ * We don't use sizeof(struct page) directly since taking its size here
+ * requires its definition to be available at this point in the inclusion
+ * chain, and it may not be a power of 2 in the first place.
+ */
+#define STRUCT_PAGE_MAX_SHIFT	6

I know this is copied from arm64, but wouldn't this be a good time
to move this next to the struct page defintion?

Also this:

arch/arm64/mm/init.c:   BUILD_BUG_ON(sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT));

should move to comment code (or would have to be duplicated for riscv)

Makes sense. Where is a good place for the BUILD_BUG_ON in common code?

Never mind. Seems like it's pretty trivial to do this:

#define STRUCT_PAGE_MAX_SHIFT \
    ilog2(roundup_pow_of_two(sizeof(struct page)))

So the BUILD_BUG_ON becomes unnecessary.

The comment saying it can't be done is really misleading as it wasn't
actually difficult.

Logan