• Yongseok Koh's avatar
    vmalloc: remove BUG_ON due to racy counting of VM_LAZY_FREE · 88f50044
    Yongseok Koh authored
    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
    
    [ See also http://marc.info/?l=linux-kernel&m=126335856228090&w=2
    
     ]
    Signed-off-by: default avatarYongseok Koh <yongseok.koh@samsung.com>
    Reviewed-by: default avatarMinchan Kim <minchan.kim@gmail.com>
    Cc: Nick Piggin <npiggin@suse.de>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    88f50044
vmalloc.c 59.4 KB