On Fri, 31 Oct 2008, FUJITA Tomonori wrote: > On Fri, 31 Oct 2008 03:12:11 -0400 (EDT) > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > I looked at your implementation and I have these comments on it. > > Thanks for the reviewing! > > btw, can we discuss this on the mailing list? Some developers might be > interested in the progress. I see. I CC'd mailing list. > > 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. > 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. > The shared-exception-snapshot target wants to cache chunks and update > multiple chunks atomically. > > > > How do you handle the situation when the user removes origin with dmsetup, > > but leaves snapshots there? Don't you access already closed devices if > > that happens? This is kind of tricky to solve, because: > > - there is no way to prevent removal of a target in device mapper > > architecture. So you can't say "I refuse to remove origin if I have open > > snapshots". > > - open devices are bound to dm_target structure, so you couldn't open > > something in origin constructor, hand it over to snapshot and close it in > > snapshot destructor. > > The only solution I see for this problem is: when destructing the origin, > > mark snapshots as unusable (with some flag, so that they always return > > -EIO), wait for all IOs on the snapshots to drain and then procceed with > > closing the origin. > > If you have some other ideas, how to solve this, say them. > > Yeah, this issue is on my todo list. I've not investigated this yet. > > 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. > 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. If you want to use dm-bufio, you have to either change the btree code so that it holds at most one buffer (i.e. no more brelse_path). Another possiblity would be that I change dm-bufio for you to have more reserved buffers --- but then, you must guarantee that only one thread can take buffers. If you had multiple threads, each taking multiple buffers, there could be an out-of-memory deadlock. Mikulas > Thanks! > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
