[PATCH 06/32] sched: Add task_struct->faults_disabled_mapping

Kent Overstreet
2023-05-09T16:57:54Z

From: Kent Overstreet <kent.overstreet@gmail.com>

This is used by bcachefs to fix a page cache coherency issue with
O_DIRECT writes.

Also relevant: mapping->invalidate_lock, see below.

O_DIRECT writes (and other filesystem operations that modify file data
while bypassing the page cache) need to shoot down ranges of the page
cache - and additionally, need locking to prevent those pages from
pulled back in.

But O_DIRECT writes invoke the page fault handler (via get_user_pages),
and the page fault handler will need to take that same lock - this is a
classic recursive deadlock if userspace has mmaped the file they're DIO
writing to and uses those pages for the buffer to write from, and it's a
lock ordering deadlock in general.

Thus we need a way to signal from the dio code to the page fault handler
when we already are holding the pagecache add lock on an address space -
this patch just adds a member to task_struct for this purpose. For now
only bcachefs is implementing this locking, though it may be moved out
of bcachefs and made available to other filesystems in the future.

---------------------------------

The closest current VFS equivalent is mapping->invalidate_lock, which
comes from XFS. However, it's not used by direct IO. Instead, direct IO
paths shoot down the page cache twice - before starting the IO and at
the end, and they're still technically racy w.r.t. page cache coherency.

This is a more complete approach: in the future we might consider
replacing mapping->invalidate_lock with the bcachefs code.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Jan Kara <jack@suse.cz>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
---
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 63d242164b..f2a56f64f7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -869,6 +869,7 @@ 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 ff6c4b9bfe..f703116e05 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,
},
--
2.40.1