On Sat, Oct 09, 2010 at 02:12:24AM +0200, Jan Kara wrote: > > currently, when mmapped write is done to a file backed by ext3, the > filesystem does nothing to make sure blocks will be available when we need > to write them out. Hmm, you've done all of this work already, so this isn't the best time to suggest this, but I wonder if we've explored all of the alternatives that might allow for a less drastic set of changes to ext3, just out of stability's sake. How often do legitimate workloads mmap a sparse file then write into it? As I recall, the original POSIX.1 spec didn't allow mmap beyond the end of the file; this I believe was lifted later on (at least I don't see it in SUSv3 spec). If it's not all that common, then other options are: 1) Fail an mmap with EINVAL if there is an attempt to map a file region which is either sparse or extends beyond the end of a file. This is probably not a great alternative, but it's a possibility. 2) Allocate all of the pages that are not allocated at mmap time. Since ext3 doesn't have space for an uninitialized bit, we'd have to either (2a) forcing a disk write out for all of the newly initialized pages, or (2b) keep track of the allocated disk blocks in memory, but don't actually write the block mappings to the indirect blocks until the blocks are actually written out. (This last might be just as complex, alas). 3) Keep a global counter of sparse blocks which are mapped at mmap() time, and update it as blocks are allocated, or when the region is freed at munmap() time. #3 might be much simpler, at the end of the day. Note that there are some Japanese customers that really freaked with ext4 just because it was *different*, and begged a distribution not to ship ext4 because it might destablize their customers. Not that I think we are obliged to listen to some of the more extremely conservative customers, but there was something nice about telling people (well, if you want something which is nice and stable and conservative, you can pick ext3). Do really have legitimate and common workloads which are allocating blocks by writing into an mmapped region? I wasn't aware of such beasts, but maybe they are out there... - 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
