On Fri, 31 Oct 2008 08:27:26 -0400 (EDT) Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > You have alloc_chunk_buffer in alloc_chunk_buffer. vmalloc can trigger > > > writeouts and wait for them, resulting in a deadlock. Try > > > __vmalloc(size, GFP_NOFS, PAGE_KERNEL) > > > instead. Or you can use bufio instead of this, see below. > > Oh, now I realized that it should be GFP_NOIO, not GFP_NOFS (both here and > in kzalloc). GFP_NOFS causes that the allocator won't reenter filesystem, > but it can still wait on IO on pages or swap. GFP_NOIO supresses any IO > requests from the allocator. > > Please, also rename brelse and set_buffer_dirty to something else, because > these functions already exists in Linux --- they're static in your > implementation, but if structure of include files change, you could easily > end up including that one that contains brelse and set_buffer_dirty and > get an error from conflicting definitions. They came from Zumastor. I'm too lazy to change these names. Well, I'll change them later. > > If dm provides something that can work for the > > shared-exception-snapshot target, I'd love to use it and dump the > > home-grown caching code. > > The core dm doesn't provide it now, but my patch (into core dm) does. The > patch would be merged one day into core dm. Well, I can use only things in mainline or things will be surely merged into mainline. > > Your solution is fine by me. If an admin removes the origin, all the > > snapshots of it has to go. It sounds reasonable. Well, if we add a new > > mechanism for the first option to dm, that might be better. > > I thought about it, but preventing removal of target would affect some > other code (for example, dmsetup remove_all command would have to remove > targets according to their dependencies), so it'd be better just solve > this problem in a target and do not try to change device mapper > architecture. As I said, it would needs lots of changes, let's go with other options. > > Anyway, I'd like to go with the simplest option. As long as the kernel > > doesn't crash, I have no complaint. > > > > > You can look at bufio layer, I wrote it for my implementation: > > > http://people.redhat.com/mpatocka/patches/kernel/2.6.27/dm-bufio.patch > > > Bufio does exactly the same thing as your caching, but it is implemented > > > in such a way that multiple targets can reuse it. So it could be good to > > > use it to reduce code duplication. > > > > > > With bufio, you can hold at most one buffer per thread, more threads can > > > hold buffers concurrently. The buffer is accessed with dm_bufio_read > > > (reads from disk) or dm_bufio_new (doesn't read from disk, it is expected > > > that the caller immediatelly initializes the buffer). After access, it is > > > released with dm_bufio_release. dm_bufio_mark_buffer_dirty marks the > > > buffer dirty after you modified it and dm_bufio_write_dirty_buffers writes > > > dirty buffers. Dirty buffers can be written automatically prior to > > > dm_bufio_write_dirty_buffers if there is memory pressure. If you want to > > > use it and have some comments on it (or you need some extensions), write > > > to me. > > > > As I wrote above, I'm pretty happy to try bufio if it will be merged > > into mainline. I guess that bufio doesn't update multiple chunks > > atomically but it's fine. I can work on it later on. > > Well, nothing can update multiple chunks atomically :) The disks only > guarantee you that they update 512-byte sector atomically, not even the > whole chunk. You have to use journaling or some other method (phase trees, > crash counts...) to get atomic updates --- and you can use it with > dm-bufio. As I said in the first submission, I plan to use journaling. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
