Re: "data=writeback" and TRIM don't get along

[Eric Sandeen <sandeen@redhat.com>]

Eric Sandeen wrote:
> I'll have to think about the right way to do this... it seems pretty
> convoluted to me right now.
>   
Something like this probably works, but I really REALLY would not test
it on an important filesystem.   :)

I'm not sure it's a good idea to discard it before returning it
to the prealloc pool, because it may well get re-used again
quickly.... not sure if that's helpful.

Just a note, I think eventually we may move to more of a batch discard
in the background, because these little discards are actually quite
inefficient on the hardware we've tested so far.

-Eric

p.s. really.  Don't test this with important data.  I haven't tested
it at all yet.

Index: linux-2.6/fs/ext4/mballoc.c
===================================================================
--- linux-2.6.orig/fs/ext4/mballoc.c
+++ linux-2.6/fs/ext4/mballoc.c
@@ -4602,6 +4606,8 @@ do_more:
 		mb_clear_bits(bitmap_bh->b_data, bit, count);
 		ext4_mb_free_metadata(handle, &e4b, new_entry);
 	} else {
+		ext4_fsblk_t discard_block;
+
 		/* need to update group_info->bb_free and bitmap
 		 * with group lock held. generate_buddy look at
 		 * them with group lock_held
@@ -4609,6 +4615,11 @@ do_more:
 		ext4_lock_group(sb, block_group);
 		mb_clear_bits(bitmap_bh->b_data, bit, count);
 		mb_free_blocks(inode, &e4b, bit, count);
+		discard_block = bit +
+				ext4_group_first_block_no(sb, block_group);
+		trace_ext4_discard_blocks(sb,
+				(unsigned long long)discard_block, count);
+		sb_issue_discard(sb, discard_block, count);
 		ext4_mb_return_to_preallocation(inode, &e4b, block, count);
 	}