On Sun, Mar 06, 2011 at 10:15:41PM +0200, Amir Goldstein wrote: > > That sounds about right, but why do I need a new bit? > Why can't I use EXT4_GROUP_INFO_NEED_INIT_BIT to tell me the exact > same thing? The current meaning of NEED_INIT_BIT is that it indicates that the group has been initialized once since the file system has been mounted. It is used by ext4_mb_good_group() to know whether it can rely on ext4_group_info->bb_free, ext4_group_info->bb_fragments, ext4_group_info->bb_largest_free_order, et. al, without needing to reload the buddy bitmap. We added this so that even if memory pressure has forced the buddy bitmap and block allocation bitmaps out of memory, we have enough information in the ext4_group_info summary array that we can quickly decide whether or not a group is a likely good candidate to be examined more closely to have the necessary free blocks. Without this (relatively recent) change, the mballoc code might potentially need to read in tens if not hundreds of block allocation bitmaps only to find that it didn't have enough contiguous blocks, and then the memory pressure would push the block bitmap out of memory again.... and file system performance would go into the toilet. So we don't want to disturb the meaning of this particular bit. If we zap the NEED_INIT_BIT whenever we discover that the group's buddy bitmap page has been pushed out of memory, then we will once again need to read in massive numbers of block bitmaps because clearing the bit effectively marks the summary information stored ext4_group_info structure as invalid. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
