From: Miklos Szeredi <mszeredi@xxxxxxx> This deadlock happens, when dirty pages from one filesystem are written back through another filesystem. It easiest to demonstrate with fuse although it could affect looback mounts as well (see following patches). Let's call the filesystems A(bove) and B(elow). Process Pr_a is writing to A, and process Pr_b is writing to B. Pr_a is bash-shared-mapping. Pr_b is the fuse filesystem daemon (fusexmp_fh), for simplicity let's assume that Pr_b is single threaded. These are the simplified stack traces of these processes after the deadlock: Pr_a (bash-shared-mapping): (block on queue) fuse_writepage generic_writepages writeback_inodes balance_dirty_pages balance_dirty_pages_ratelimited_nr set_page_dirty_mapping_balance do_no_page Pr_b (fusexmp_fh): io_schedule_timeout congestion_wait balance_dirty_pages balance_dirty_pages_ratelimited_nr generic_file_buffered_write generic_file_aio_write ext3_file_write do_sync_write vfs_write sys_pwrite64 Thanks to the aggressive nature of Pr_a, it can happen, that nr_file_dirty > dirty_thresh + margin This is due to both nr_dirty growing and dirty_thresh shrinking, which in turn is due to nr_file_mapped rapidly growing. The exact size of the margin at which the deadlock happens is not known, but it's around 100 pages. At this point Pr_a enters balance_dirty_pages and starts to write back some if it's dirty pages. After submitting some requests, it blocks on the request queue. The first write request will trigger Pr_b to perform a write() syscall. This will submit a write request to the block device and then may enter balance_dirty_pages(). The condition for exiting balance_dirty_pages() is - either that write_chunk pages have been written - or nr_file_dirty + nr_writeback < dirty_thresh It is entirely possible that less than write_chunk pages were written, in which case balance_dirty_pages() will not exit even after all the submitted requests have been succesfully completed. Which means that the write() syscall does not return. Which means, that no more dirty pages from A will be written back, and neither nr_writeback nor nr_file_dirty will decrease. Which means, that balance_dirty_pages() will loop forever. What if Pr_b is multithreaded? The first thread will enter balance_dirty_pages() and loop there as shown above. It will hold i_mutex for the inode, taken in generic_file_aio_write(). The other theads now try to write back more data into the same file, but will block on i_mutex. So even with unlimited number of threads no progress is made. Q.E.D. The solution is to exit balance_dirty_pages() on the condition, that there are only a few dirty + writeback pages for this backing dev. This makes sure, that there is always some progress with this setup. The number of outstanding dirty + written pages is limited to 8, which means that when over the threshold (dirty_exceeded == 1), each filesystem may only effectively pin a maximum of 16 (+8 because of ratelimiting) extra pages. Note: a similar safety vent is always needed if there's a global limit for the dirty+writeback pages, even if in the future there will be some per-queue (or other) soft limit. Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx> --- Index: linux/mm/page-writeback.c =================================================================== --- linux.orig/mm/page-writeback.c 2007-02-27 14:41:07.000000000 +0100 +++ linux/mm/page-writeback.c 2007-02-27 14:41:07.000000000 +0100 @@ -201,6 +201,17 @@ static void balance_dirty_pages(struct a if (!dirty_exceeded) dirty_exceeded = 1; + /* + * Acquit producer of dirty pages if there's little or + * nothing to write back to this particular queue. + * + * Without this check a deadlock is possible for if + * one filesystem is writing data through another. + */ + if (atomic_long_read(&bdi->nr_dirty) + + atomic_long_read(&bdi->nr_writeback) < 8) + break; + /* Note: nr_reclaimable denotes nr_dirty + nr_unstable. * Unstable writes are a feature of certain networked * filesystems (i.e. NFS) in which data may have been -- - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
