Re: [RFC PATCH] Add proc interface to set PF_MEMALLOC flags

[Hillf Danton <hdanton@sina.com>]

On Tue, 10 Sep 2019 11:06:03 -0500 From: Mike Christie <mchristi@redhat.com>
>
> > Really? Without any privilege check? So any random user can tap into
> > __GFP_NOIO allocations?
>
> That was a mistake on my part. I will add it in.
>
You may alternatively madvise a nutcracker as long as you would have
added a sledgehammer under /proc instead of a gavel.

--- a/include/uapi/asm-generic/mman-common.h
+++ b/include/uapi/asm-generic/mman-common.h
@@ -45,6 +45,7 @@
 #define MADV_SEQUENTIAL	2		/* expect sequential page references */
 #define MADV_WILLNEED	3		/* will need these pages */
 #define MADV_DONTNEED	4		/* don't need these pages */
+#define MADV_NOIO	5		/* set PF_MEMALLOC_NOIO */
 
 /* common parameters: try to keep these consistent across architectures */
 #define MADV_FREE	8		/* free pages only if memory pressure */
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -716,6 +716,7 @@ madvise_behavior_valid(int behavior)
 	case MADV_WILLNEED:
 	case MADV_DONTNEED:
 	case MADV_FREE:
+	case MADV_NOIO:
 #ifdef CONFIG_KSM
 	case MADV_MERGEABLE:
 	case MADV_UNMERGEABLE:
@@ -813,6 +814,11 @@ SYSCALL_DEFINE3(madvise, unsigned long,
 	if (!madvise_behavior_valid(behavior))
 		return error;
 
+	if (behavior == MADV_NOIO) {
+		current->flags |= PF_MEMALLOC_NOIO;
+		return 0;
+	}
+
 	if (start & ~PAGE_MASK)
 		return error;
 	len = (len_in + ~PAGE_MASK) & PAGE_MASK;