Re: [PATCH 1/4] mm/vmalloc: allow to call vfree() in atomic context

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 30 Mar 2017 13:27:16 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:

> 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.

I tend to disagree: adding yet another schedule_work() introduces
additional overhead and adds some risk of ENOMEM errors which wouldn't
occur with a synchronous free.

> __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.

vfree() already does

	if (unlikely(in_interrupt()))
		__vfree_deferred(addr);

so it seems silly to introduce another defer-to-kernel-thread thing
when we already have one.

> This will have a minimal effect on the regular vfree() path.
> since __purge_vmap_area_lazy() is rarely called.

hum, OK, so perhaps the overhead isn't too bad.

Remind me: where does __purge_vmap_area_lazy() sleep?


Seems to me that a better fix would be to make vfree() atomic, if poss.

Otherwise, to fix callers so they call vfree from sleepable context. 
That will reduce kernel latencies as well.

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