Re: [PATCH] vmalloc: remove BUG_ON due to racy counting of VM_LAZY_FREE

Minchan Kim
2010-01-19T12:01:38Z

On Tue, 2010-01-19 at 17:33 +0900, Yongseok Koh wrote:
> From: Yongseok Koh <yongseok.koh@samsung.com>

You don't need above line.
We use "From" when we send patch instead of someone.

>
> In free_unmap_area_noflush(), va->flags is marked as VM_LAZY_FREE first, and
> then vmap_lazy_nr is increased atomically.
> But, in __purge_vmap_area_lazy(), while traversing of vmap_are_list, nr is
> counted by checking VM_LAZY_FREE is set to va->flags.
> After counting the variable nr, kernel reads vmap_lazy_nr atomically and
> checks a BUG_ON condition whether nr is greater than vmap_lazy_nr to prevent
> vmap_lazy_nr from being negative.
>
> The problem is that, if interrupted right after marking VM_LAZY_FREE,
> increment of vmap_lazy_nr can be delayed.
> Consequently, BUG_ON condition can be met because nr is counted more than
> vmap_lazy_nr.
>
> It is highly probable when vmalloc/vfree are called frequently.
> This scenario have been verified by adding delay between marking
> VM_LAZY_FREE and increasing vmap_lazy_nr in free_unmap_area_noflush().
>
> Even the vmap_lazy_nr is for checking high watermark, it never be the strict
> watermark.
> Although the BUG_ON condition is to prevent vmap_lazy_nr from being
> negative, vmap_lazy_nr is signed variable.
> So, it could go down to negative value temporarily.
>
> Consequently, removing the BUG_ON condition is proper.
>
> A possible BUG_ON message is like the below.
>
> kernel BUG at mm/vmalloc.c:517!
> invalid opcode: 0000 [#1] SMP
> EIP: 0060:[<c04824a4>] EFLAGS: 00010297 CPU: 3
> EIP is at __purge_vmap_area_lazy+0x144/0x150
> EAX: ee8a8818 EBX: c08e77d4 ECX: e7c7ae40 EDX: c08e77ec
> ESI: 000081fe EDI: e7c7ae60 EBP: e7c7ae64 ESP: e7c7ae3c
> DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
> Call Trace:
> [<c0482ad9>] free_unmap_vmap_area_noflush+0x69/0x70
> [<c0482b02>] remove_vm_area+0x22/0x70
> [<c0482c15>] __vunmap+0x45/0xe0
> [<c04831ec>] vmalloc+0x2c/0x30
> Code: 8d 59 e0 eb 04 66 90 89 cb 89 d0 e8 87 fe ff ff 8b 43 20 89 da 8d 48
> e0 8d 43 20 3b 04 24 75 e7 fe 05 a8 a5 a3 c0 e9 78 ff ff ff <0f> 0b eb fe 90
> 8d b4 26 00 00 00 00 56 89 c6 b8 ac a5 a3 c0 31
> EIP: [<c04824a4>] __purge_vmap_area_lazy+0x144/0x150 SS:ESP 0068:e7c7ae3c
>
>
> Signed-off-by: Yongseok Koh <yongseok.koh@samsung.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

We discussed about this following as.
http://marc.info/?l=linux-kernel&m=126335856228090&w=2

Thanks for contribution for linux kernel, Yongseok. :)

--
Kind regards,
Minchan Kim