Commit 4c0cfebd authored by Theodore Ts'o's avatar Theodore Ts'o
Browse files

ext4: clean up mballoc criteria comments 



Line wrap and slightly clarify the comments describing mballoc's
cirtiera.

Define EXT4_MB_NUM_CRS as part of the enum, so that it will
automatically get updated when criteria is added or removed.

Also fix a potential unitialized use of 'cr' variable if
CONFIG_EXT4_DEBUG is enabled.

Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent ab8627e1

fs/ext4/ext4.h

+25 −20
Original line number Diff line number Diff line
@@ -128,47 +128,52 @@ enum SHIFT_DIRECTION { 
};



/*

 * Number of criterias defined. For each criteria, mballoc has slightly

 * different way of finding the required blocks nad usually, higher the

 * criteria the slower the allocation. We start at lower criterias and keep

 * falling back to higher ones if we are not able to find any blocks.

 */

#define EXT4_MB_NUM_CRS 5

/*

 * All possible allocation criterias for mballoc. Lower are faster.

 * For each criteria, mballoc has slightly different way of finding

 * the required blocks nad usually, higher the criteria the slower the

 * allocation.  We start at lower criterias and keep falling back to

 * higher ones if we are not able to find any blocks.  Lower (earlier)

 * criteria are faster.

 */

enum criteria {

	/*

	 * Used when number of blocks needed is a power of 2. This doesn't

	 * trigger any disk IO except prefetch and is the fastest criteria.

	 * Used when number of blocks needed is a power of 2. This

	 * doesn't trigger any disk IO except prefetch and is the

	 * fastest criteria.

	 */

	CR_POWER2_ALIGNED,



	/*

	 * Tries to lookup in-memory data structures to find the most suitable

	 * group that satisfies goal request. No disk IO except block prefetch.

	 * Tries to lookup in-memory data structures to find the most

	 * suitable group that satisfies goal request. No disk IO

	 * except block prefetch.

	 */

	CR_GOAL_LEN_FAST,



        /*

	 * Same as CR_GOAL_LEN_FAST but is allowed to reduce the goal length to

         * the best available length for faster allocation.

	 * Same as CR_GOAL_LEN_FAST but is allowed to reduce the goal

         * length to the best available length for faster allocation.

	 */

	CR_BEST_AVAIL_LEN,



	/*

	 * Reads each block group sequentially, performing disk IO if necessary, to

	 * find find_suitable block group. Tries to allocate goal length but might trim

	 * the request if nothing is found after enough tries.

	 * Reads each block group sequentially, performing disk IO if

	 * necessary, to find find_suitable block group. Tries to

	 * allocate goal length but might trim the request if nothing

	 * is found after enough tries.

	 */

	CR_GOAL_LEN_SLOW,



	/*

	 * Finds the first free set of blocks and allocates those. This is only

	 * used in rare cases when CR_GOAL_LEN_SLOW also fails to allocate

	 * anything.

	 * Finds the first free set of blocks and allocates

	 * those. This is only used in rare cases when

	 * CR_GOAL_LEN_SLOW also fails to allocate anything.

	 */

	CR_ANY_FREE,



	/*

	 * Number of criterias defined.

	 */

	EXT4_MB_NUM_CRS

};



/* criteria below which we use fast block scanning and avoid unnecessary IO */

fs/ext4/mballoc.c

+9 −10
Original line number Diff line number Diff line
@@ -1035,11 +1035,9 @@ static void ext4_mb_choose_next_group_best_avail(struct ext4_allocation_context 


		if (num_stripe_clusters > 0) {

			/*

			 * Try to round up the adjusted goal to stripe size

			 * (in cluster units) multiple for efficiency.

			 *

			 * XXX: Is s->stripe always a power of 2? In that case

			 * we can use the faster round_up() variant.

			 * Try to round up the adjusted goal length to

			 * stripe size (in cluster units) multiple for

			 * efficiency.

			 */

			ac->ac_g_ex.fe_len = roundup(ac->ac_g_ex.fe_len,

						     num_stripe_clusters);
@@ -2758,7 +2756,7 @@ static noinline_for_stack int 
ext4_mb_regular_allocator(struct ext4_allocation_context *ac)

{

	ext4_group_t prefetch_grp = 0, ngroups, group, i;

	enum criteria cr, new_cr;

	enum criteria new_cr, cr = CR_GOAL_LEN_FAST;

	int err = 0, first_err = 0;

	unsigned int nr = 0, prefetch_ios = 0;

	struct ext4_sb_info *sbi;
@@ -2815,12 +2813,13 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac) 
		spin_unlock(&sbi->s_md_lock);

	}



	/* Let's just scan groups to find more-less suitable blocks */

	cr = ac->ac_2order ? CR_POWER2_ALIGNED : CR_GOAL_LEN_FAST;

	/*

	 * cr == CR_POWER2_ALIGNED try to get exact allocation,

	 * cr == CR_ANY_FREE try to get anything

	 * Let's just scan groups to find more-less suitable blocks We

	 * start with CR_GOAL_LEN_FAST, unless it is power of 2

	 * aligned, in which case let's do that faster approach first.

	 */

	if (ac->ac_2order)

		cr = CR_POWER2_ALIGNED;

repeat:

	for (; cr < EXT4_MB_NUM_CRS && ac->ac_status == AC_STATUS_CONTINUE; cr++) {

		ac->ac_criteria = cr;