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