Re: [PATCH v2 11/12] ext4: Add allocation criteria 1.5 (CR1_5)

["Theodore Ts'o" <tytso@mit.edu>]

Jan, thanks for the comments to Ojaswin's patch series.  Since I had
already landed his patch series in my tree and have been testing it,
I've fixed the obvious issues you've raised in a fixup patch
(attached).

There is one issue which I have not fixed:

On Wed, Jun 07, 2023 at 12:21:03PM +0200, Jan Kara wrote:
> > +	for (i = order; i >= min_order; i--) {
> > +		int frag_order;
> > +		/*
> > +		 * Scale down goal len to make sure we find something
> > +		 * in the free fragments list. Basically, reduce
> > +		 * preallocations.
> > +		 */
> > +		ac->ac_g_ex.fe_len = 1 << i;
> 
> I smell some off-by-one issues here. Look fls(1) == 1 so (1 << fls(n)) > n.
> Hence this loop will actually *grow* the goal allocation length. Also I'm
> not sure why you have +1 in min_order = fls(ac->ac_o_ex.fe_len) + 1.

Ojaswin, could you take a look this?  Thanks!!

	       	   	       	      - Ted

commit 182d2d90a180838789ed5a19e08c333043d1617a
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Thu Jun 8 10:39:35 2023 -0400

    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: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 6a1f013d23f7..45a531446ea2 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -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 */
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 8a6896d4e9b0..2f9f5dc720cc 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2759,7 +2759,7 @@ 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;
@@ -2816,12 +2816,13 @@ 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;