btrfs: increase the metadata allowance for the free_space_cache

#include "discard.h"

#define BITS_PER_BITMAP		(PAGE_SIZE * 8UL)

#define MAX_CACHE_BYTES_PER_GIG	SZ_32K

#define MAX_CACHE_BYTES_PER_GIG	SZ_64K

#define FORCE_EXTENT_THRESHOLD	SZ_1M

struct btrfs_trim_range {

	u64 start;

	ASSERT(ctl->total_bitmaps <= max_bitmaps);

	/*

	 * The goal is to keep the total amount of memory used per 1gb of space

	 * at or below 32k, so we need to adjust how much memory we allow to be

	 * used by extent based free space tracking

	 * We are trying to keep the total amount of memory used per 1GiB of

	 * space to be MAX_CACHE_BYTES_PER_GIG.  However, with a reclamation

	 * mechanism of pulling extents >= FORCE_EXTENT_THRESHOLD out of

	 * bitmaps, we may end up using more memory than this.

	 */

	if (size < SZ_1G)

		max_bytes = MAX_CACHE_BYTES_PER_GIG;

	else

		max_bytes = MAX_CACHE_BYTES_PER_GIG * div_u64(size, SZ_1G);

	/*

	 * we want to account for 1 more bitmap than what we have so we can make

	 * sure we don't go over our overall goal of MAX_CACHE_BYTES_PER_GIG as

	 * we add more bitmaps.

	 */

	bitmap_bytes = (ctl->total_bitmaps + 1) * ctl->unit;

	if (bitmap_bytes >= max_bytes) {

		ctl->extents_thresh = 0;

		return;

	}

	bitmap_bytes = ctl->total_bitmaps * ctl->unit;

	/*

	 * we want the extent entry threshold to always be at most 1/2 the max

		forced = true;

#endif

	/* This is a way to reclaim large regions from the bitmaps. */

	if (!forced && info->bytes >= FORCE_EXTENT_THRESHOLD)

		return false;

	/*

	 * If we are below the extents threshold then we can add this as an

	 * extent, and don't have to deal with the bitmap