libceph: avoid a __vmalloc() deadlock in ceph_kvmalloc()

The vmalloc allocator doesn't fully respect the specified gfp mask:
while the actual pages are allocated as requested, the page table pages
are always allocated with GFP_KERNEL.  ceph_kvmalloc() may be called
with GFP_NOFS and GFP_NOIO (for ceph and rbd respectively), so this may
result in a deadlock.

There is no real reason for the current PAGE_ALLOC_COSTLY_ORDER logic,
it's just something that seemed sensible at the time (ceph_kvmalloc()
predates kvmalloc()).  kvmalloc() is smarter: in an attempt to reduce
long term fragmentation, it first tries to kmalloc non-disruptively.

Switch to kvmalloc() and set the respective PF_MEMALLOC_* flag using
the scope API to avoid the deadlock.  Note that kvmalloc() needs to be
passed GFP_KERNEL to enable the fallback.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

net/ceph/ceph_common.c

@@ -13,6 +13,7 @@
 #include <linux/nsproxy.h>
 #include <linux/parser.h>
 #include <linux/sched.h>
+#include <linux/sched/mm.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/statfs.h>
@@ -185,18 +186,34 @@ int ceph_compare_options(struct ceph_options *new_opt,
 }
 EXPORT_SYMBOL(ceph_compare_options);
 
+/*
+ * kvmalloc() doesn't fall back to the vmalloc allocator unless flags are
+ * compatible with (a superset of) GFP_KERNEL.  This is because while the
+ * actual pages are allocated with the specified flags, the page table pages
+ * are always allocated with GFP_KERNEL.  map_vm_area() doesn't even take
+ * flags because GFP_KERNEL is hard-coded in {p4d,pud,pmd,pte}_alloc().
+ *
+ * ceph_kvmalloc() may be called with GFP_KERNEL, GFP_NOFS or GFP_NOIO.
+ */
 void *ceph_kvmalloc(size_t size, gfp_t flags)
 {
-	if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)) {
-		void *ptr = kmalloc(size, flags | __GFP_NOWARN);
-		if (ptr)
-			return ptr;
+	void *p;
+
+	if ((flags & (__GFP_IO | __GFP_FS)) == (__GFP_IO | __GFP_FS)) {
+		p = kvmalloc(size, flags);
+	} else if ((flags & (__GFP_IO | __GFP_FS)) == __GFP_IO) {
+		unsigned int nofs_flag = memalloc_nofs_save();
+		p = kvmalloc(size, GFP_KERNEL);
+		memalloc_nofs_restore(nofs_flag);
+	} else {
+		unsigned int noio_flag = memalloc_noio_save();
+		p = kvmalloc(size, GFP_KERNEL);
+		memalloc_noio_restore(noio_flag);
 	}
 
-	return __vmalloc(size, flags, PAGE_KERNEL);
+	return p;
 }
 
-
 static int parse_fsid(const char *str, struct ceph_fsid *fsid)
 {
 	int i = 0;