mm: Add a mechanism to disable faults for a specific mapping

This will be used to prevent a nasty cache coherency issue for O_DIRECT
writes; O_DIRECT writes need to shoot down the range of the page cache
corresponding to the part of the file being written to - but, if the
file is mapped in, userspace can pass in an address in that mapping to
pwrite(), causing those pages to be faulted back into the page cache
in get_user_pages().

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

3 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c69f308f3a53..96e4152cc9bc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -687,6 +687,7 @@ struct task_struct {
 
 	struct mm_struct		*mm;
 	struct mm_struct		*active_mm;
+	struct address_space		*faults_disabled_mapping;
 
 	/* Per-thread vma caching: */
 	struct vmacache			vmacache;
diff --git a/init/init_task.c b/init/init_task.c
index 5aebe3be4d7c..2a275774eb52 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -75,6 +75,7 @@ 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,
 	},
diff --git a/mm/gup.c b/mm/gup.c
index f3088d25bd92..c1603a71047c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -724,6 +724,12 @@ retry:
 		if (unlikely(fatal_signal_pending(current)))
 			return i ? i : -ERESTARTSYS;
 		cond_resched();
+
+		if (current->faults_disabled_mapping &&
+		    vma->vm_file &&
+		    vma->vm_file->f_mapping == current->faults_disabled_mapping)
+			return -EFAULT;
+
 		page = follow_page_mask(vma, start, foll_flags, &page_mask);
 		if (!page) {
 			int ret;