From 2b69987be575b92adb6c177679f3c559134f0d8f Mon Sep 17 00:00:00 2001 From: Kent Overstreet Date: Wed, 16 Oct 2019 15:03:50 -0400 Subject: sched: Add task_struct->faults_disabled_mapping MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit There has been a long standing page cache coherence bug with direct IO. This provides part of a mechanism to fix it, currently just used by bcachefs but potentially worth promoting to the VFS. Direct IO evicts the range of the pagecache being read or written to. For reads, we need dirty pages to be written to disk, so that the read doesn't return stale data. For writes, we need to evict that range of the pagecache so that it's not stale after the write completes. However, without a locking mechanism to prevent those pages from being re-added to the pagecache - by a buffered read or page fault - page cache inconsistency is still possible. This isn't necessarily just an issue for userspace when they're playing games; filesystems may hang arbitrary state off the pagecache, and so page cache inconsistency may cause real filesystem bugs, depending on the filesystem. This is less of an issue for iomap based filesystems, but e.g. buffer heads caches disk block mappings (!) and attaches them to the pagecache, and bcachefs attaches disk reservations to pagecache pages. This issue has been hard to fix, because - we need to add a lock (henceforth called pagecache_add_lock), which would be held for the duration of the direct IO - page faults add pages to the page cache, thus need to take the same lock - dio -> gup -> page fault thus can deadlock And we cannot enforce a lock ordering with this lock, since userspace will be controlling the lock ordering (via the fd and buffer arguments to direct IOs), so we need a different method of deadlock avoidance. We need to tell the page fault handler that we're already holding a pagecache_add_lock, and since plumbing it through the entire gup() path would be highly impractical this adds a field to task_struct. Then the full method is: - in the dio path, when we first take the pagecache_add_lock, note the mapping in the current task_struct - in the page fault handler, if faults_disabled_mapping is set, we check if it's the same mapping as the one we're taking a page fault for, and if so return an error. Then we check lock ordering: if there's a lock ordering violation and trylock fails, we'll have to cycle the locks and return an error that tells the DIO path to retry: faults_disabled_mapping is also used for signalling "locks were dropped, please retry". Also relevant to this patch: mapping->invalidate_lock. mapping->invalidate_lock provides most of the required semantics - it's used by truncate/fallocate to block pages being added to the pagecache. However, since it's a rwsem, direct IOs would need to take the write side in order to block page cache adds, and would then be exclusive with each other - we'll need a new type of lock to pair with this approach. Signed-off-by: Kent Overstreet Cc: Jan Kara Cc: Darrick J. Wong Cc: linux-fsdevel@vger.kernel.org Cc: Andreas Grünbacher --- include/linux/sched.h | 1 + init/init_task.c | 1 + 2 files changed, 2 insertions(+) diff --git a/include/linux/sched.h b/include/linux/sched.h index 77f01ac385f7..d5951e99706a 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -875,6 +875,7 @@ struct task_struct { struct mm_struct *mm; struct mm_struct *active_mm; + struct address_space *faults_disabled_mapping; int exit_state; int exit_code; diff --git a/init/init_task.c b/init/init_task.c index ff6c4b9bfe6b..f703116e0523 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -85,6 +85,7 @@ struct task_struct init_task .nr_cpus_allowed= NR_CPUS, .mm = NULL, .active_mm = &init_mm, + .faults_disabled_mapping = NULL, .restart_block = { .fn = do_no_restart_syscall, }, -- cgit v1.2.3