[PATCH v2 1/5] mm/vmalloc: allow to call vfree() in atomic context

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

Commit 5803ed292e63 ("mm: mark all calls into the vmalloc subsystem
as potentially sleeping") added might_sleep() to remove_vm_area() from
vfree(), and commit 763b218ddfaf ("mm: add preempt points into
__purge_vmap_area_lazy()") actually made vfree() potentially sleeping.

This broke vmwgfx driver which calls vfree() under spin_lock().

    BUG: sleeping function called from invalid context at mm/vmalloc.c:1480
    in_atomic(): 1, irqs_disabled(): 0, pid: 341, name: plymouthd
    2 locks held by plymouthd/341:
     #0:  (drm_global_mutex){+.+.+.}, at: [<ffffffffc01c274b>] drm_release+0x3b/0x3b0 [drm]
     #1:  (&(&tfile->lock)->rlock){+.+...}, at: [<ffffffffc0173038>] ttm_object_file_release+0x28/0x90 [ttm]

    Call Trace:
     dump_stack+0x86/0xc3
     ___might_sleep+0x17d/0x250
     __might_sleep+0x4a/0x80
     remove_vm_area+0x22/0x90
     __vunmap+0x2e/0x110
     vfree+0x42/0x90
     kvfree+0x2c/0x40
     drm_ht_remove+0x1a/0x30 [drm]
     ttm_object_file_release+0x50/0x90 [ttm]
     vmw_postclose+0x47/0x60 [vmwgfx]
     drm_release+0x290/0x3b0 [drm]
     __fput+0xf8/0x210
     ____fput+0xe/0x10
     task_work_run+0x85/0xc0
     exit_to_usermode_loop+0xb4/0xc0
     do_syscall_64+0x185/0x1f0
     entry_SYSCALL64_slow_path+0x25/0x25

This can be fixed in vmgfx, but it would be better to make vfree()
non-sleeping again because we may have other bugs like this one.

__purge_vmap_area_lazy() is the only function in the vfree() path that
wants to be able to sleep. So it make sense to schedule
__purge_vmap_area_lazy() via schedule_work() so it runs only in sleepable
context. This will have a minimal effect on the regular vfree() path.
since __purge_vmap_area_lazy() is rarely called.

Fixes: 5803ed292e63 ("mm: mark all calls into the vmalloc subsystem as
                      potentially sleeping")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>
---
 mm/vmalloc.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 8ef8ea1..1c74b26 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -701,7 +701,7 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
  * Kick off a purge of the outstanding lazy areas. Don't bother if somebody
  * is already purging.
  */
-static void try_purge_vmap_area_lazy(void)
+static void try_purge_vmap_area_lazy(struct work_struct *work)
 {
 	if (mutex_trylock(&vmap_purge_lock)) {
 		__purge_vmap_area_lazy(ULONG_MAX, 0);
@@ -720,6 +720,8 @@ static void purge_vmap_area_lazy(void)
 	mutex_unlock(&vmap_purge_lock);
 }
 
+static DECLARE_WORK(purge_vmap_work, try_purge_vmap_area_lazy);
+
 /*
  * Free a vmap area, caller ensuring that the area has been unmapped
  * and flush_cache_vunmap had been called for the correct range
@@ -736,7 +738,7 @@ static void free_vmap_area_noflush(struct vmap_area *va)
 	llist_add(&va->purge_list, &vmap_purge_list);
 
 	if (unlikely(nr_lazy > lazy_max_pages()))
-		try_purge_vmap_area_lazy();
+		schedule_work(&purge_vmap_work);
 }
 
 /*
@@ -1125,7 +1127,6 @@ void vm_unmap_ram(const void *mem, unsigned int count)
 	unsigned long addr = (unsigned long)mem;
 	struct vmap_area *va;
 
-	might_sleep();
 	BUG_ON(!addr);
 	BUG_ON(addr < VMALLOC_START);
 	BUG_ON(addr > VMALLOC_END);
@@ -1477,8 +1478,6 @@ struct vm_struct *remove_vm_area(const void *addr)
 {
 	struct vmap_area *va;
 
-	might_sleep();
-
 	va = find_vmap_area((unsigned long)addr);
 	if (va && va->flags & VM_VM_AREA) {
 		struct vm_struct *vm = va->vm;
-- 
2.10.2