Re: [PATCH] Revalidate page->mapping in do_generic_file_read()

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Nov 4, 2010 at 7:09 AM, Dave Hansen <dave@linux.vnet.ibm.com> wrote:
>
> 70 hours into some stress tests of a 2.6.32-based enterprise kernel,
> we ran into a NULL dereference in here:
>
>        int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
>                                                unsigned long from)
>        {
> ---->           struct inode *inode = page->mapping->host;
>
> It looks like page->mapping was the culprit.  (xmon trace is below).
> After closer examination, I realized that do_generic_file_read() does
> a find_get_page(), and eventually locks the page before calling
> block_is_partially_uptodate().  However, it doesn't revalidate the
> page->mapping after the page is locked.  So, there's a small window
> between the find_get_page() and ->is_partially_uptodate() where the
> page could get truncated and page->mapping cleared.
>
> We _have_ a reference, so it can't get reclaimed, but it certainly
> can be truncated.
>
> I think the correct thing is to check page->mapping after the
> trylock_page(), and jump out if it got truncated.  This patch has
> been running in the test environment for a month or so now, and we
> have not seen this bug pop up again.
>
> xmon info:
> ======================================
> 1f:mon> e
> cpu 0x1f: Vector: 300 (Data Access) at [c0000002ae36f770]
>    pc: c0000000001e7a6c: .block_is_partially_uptodate+0xc/0x100
>    lr: c000000000142944: .generic_file_aio_read+0x1e4/0x770
>    sp: c0000002ae36f9f0
>   msr: 8000000000009032
>   dar: 0
>  dsisr: 40000000
>  current = 0xc000000378f99e30
>  paca    = 0xc000000000f66300
>    pid   = 21946, comm = bash
> 1f:mon> r
> R00 = 0025c0500000006d   R16 = 0000000000000000
> R01 = c0000002ae36f9f0   R17 = c000000362cd3af0
> R02 = c000000000e8cd80   R18 = ffffffffffffffff
> R03 = c0000000031d0f88   R19 = 0000000000000001
> R04 = c0000002ae36fa68   R20 = c0000003bb97b8a0
> R05 = 0000000000000000   R21 = c0000002ae36fa68
> R06 = 0000000000000000   R22 = 0000000000000000
> R07 = 0000000000000001   R23 = c0000002ae36fbb0
> R08 = 0000000000000002   R24 = 0000000000000000
> R09 = 0000000000000000   R25 = c000000362cd3a80
> R10 = 0000000000000000   R26 = 0000000000000002
> R11 = c0000000001e7b60   R27 = 0000000000000000
> R12 = 0000000042000484   R28 = 0000000000000001
> R13 = c000000000f66300   R29 = c0000003bb97b9b8
> R14 = 0000000000000001   R30 = c000000000e28a08
> R15 = 000000000000ffff   R31 = c0000000031d0f88
> pc  = c0000000001e7a6c .block_is_partially_uptodate+0xc/0x100
> lr  = c000000000142944 .generic_file_aio_read+0x1e4/0x770
> msr = 8000000000009032   cr  = 22000488
> ctr = c0000000001e7a60   xer = 0000000020000000   trap =  300
> dar = 0000000000000000   dsisr = 40000000
> 1f:mon> t
> [link register   ] c000000000142944 .generic_file_aio_read+0x1e4/0x770
> [c0000002ae36f9f0] c000000000142a14 .generic_file_aio_read+0x2b4/0x770
> (unreliable)
> [c0000002ae36fb40] c0000000001b03e4 .do_sync_read+0xd4/0x160
> [c0000002ae36fce0] c0000000001b153c .vfs_read+0xec/0x1f0
> [c0000002ae36fd80] c0000000001b1768 .SyS_read+0x58/0xb0
> [c0000002ae36fe30] c00000000000852c syscall_exit+0x0/0x40
> --- Exception: c00 (System Call) at 00000080a840bc54
> SP (fffca15df30) is in userspace
> 1f:mon> di c0000000001e7a6c
> c0000000001e7a6c  e9290000      ld      r9,0(r9)
> c0000000001e7a70  418200c0      beq     c0000000001e7b30        #
> .block_is_partially_uptodate+0xd0/0x100
> c0000000001e7a74  e9440008      ld      r10,8(r4)
> c0000000001e7a78  78a80020      clrldi  r8,r5,32
> c0000000001e7a7c  3c000001      lis     r0,1
> c0000000001e7a80  812900a8      lwz     r9,168(r9)
> c0000000001e7a84  39600001      li      r11,1
> c0000000001e7a88  7c080050      subf    r0,r8,r0
> c0000000001e7a8c  7f805040      cmplw   cr7,r0,r10
> c0000000001e7a90  7d6b4830      slw     r11,r11,r9
> c0000000001e7a94  796b0020      clrldi  r11,r11,32
> c0000000001e7a98  419d00a8      bgt     cr7,c0000000001e7b40    #
> .block_is_partially_uptodate+0xe0/0x100
> c0000000001e7a9c  7fa55840      cmpld   cr7,r5,r11
> c0000000001e7aa0  7d004214      add     r8,r0,r8
> c0000000001e7aa4  79080020      clrldi  r8,r8,32
> c0000000001e7aa8  419c0078      blt     cr7,c0000000001e7b20    #
> .block_is_partially_uptodate+0xc0/0x100
> ======================================
>
> Cc: arunabal@in.ibm.com
> Cc: sbest@us.ibm.com
> Cc: stable <stable@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Minchan Kim <minchan.kim@gmail.com>
> Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Nice catch.
It was much better if you comment it.
Thanks, Dave.
-- 
Kind regards,
Minchan Kim

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