On 03/19/2013 07:30 AM, Dave Chinner wrote:
> On Mon, Mar 18, 2013 at 12:48:16PM +0800, Jeff Liu wrote:
>> Hello,
>>
>> Here is the v2 patch for fixing ASSERTION failed at xfs_vm_write_failed() according
>> to Dave's comments, so I added Dave as SOB for credit.
>
> No, please don't add my SOB to a patch you wrote, even if I came up
> with the idea of how to do something. A SOB indicates that someone
> has verified the origin of the patch (e.g. for copyright reasons),
> not who contributed to finding the problem. IOWs, adding someone
> else's SOB to a patch you wrote is almost always the wrong thing to
> do.
>
> The correct way to acknowledge someone's contribution to the
> fix if they didn't write the patch is by a line in the commit
> message saying something like "Thanks to .... for helping find and
> fix the problem." [Edit: I just noticed you did this bit. ;) ]
>
> If .... agrees with your fix, then they can add a reviewed-by line
> to the patch, which has significantly different meaning to a SOB...
Ah, got it! :-P.
>
> ....
>
>> Reported-by: Michael L. Semon <mlsemon35@xxxxxxxxx>
>> Signed-off-by: Jie Liu <jeff.liu@xxxxxxxxxx>
>> Signed-off-by: Dave Chinner <david@xxxxxxxxxxxxx>
>> ---
>> fs/xfs/xfs_aops.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
>> index 5f707e5..a418e17 100644
>> --- a/fs/xfs/xfs_aops.c
>> +++ b/fs/xfs/xfs_aops.c
>> @@ -1494,7 +1494,8 @@ xfs_vm_write_failed(
>> loff_t pos,
>> unsigned len)
>> {
>> - loff_t block_offset = pos & PAGE_MASK;
>> + loff_t block_offset = (pos >> PAGE_CACHE_SHIFT) <<
>> + PAGE_CACHE_SHIFT;
>> loff_t block_start;
>> loff_t block_end;
>> loff_t from = pos & (PAGE_CACHE_SIZE - 1);
>
> This needs a comment explaining why we aren't just masking the value
> off with PAGE_MASK. And given that it wraps, something like:
>
> - loff_t block_offset = pos & PAGE_MASK;
> + loff_t block_offset;
> .....
>
> + /*
> + * comment about 32 bit systems, 64 bit variables and masks
> + */
> + block_offset = (pos >> PAGE_CACHE_SHIFT) << PAGE_CACHE_SHIFT;
Indeed, the patch is improved as following:
From: Jie Liu <jeff.liu@xxxxxxxxxx>
In xfs_vm_write_failed(), we evaluate the block_offset of pos with PAGE_MASK
which is 0xfffff000 as an unsigned long, that is fine on 64-bit platforms no
matter the request pos is 32-bit or 64-bit. However, on 32-bit platforms,
the high 32-bit in it will be masked off with (pos & PAGE_MASK) for 64-bit pos
request. As a result, the evaluated block_offset is incorrect which will cause
the ASSERT() failed: ASSERT(block_offset + from == pos);
In this case, we can get the following kernel Panic if the CONFIG_XFS_DEBUG is enabled:
[ 68.700573] XFS: Assertion failed: block_offset + from == pos, file: fs/xfs/xfs_aops.c, line: 1504
[ 68.700656] ------------[ cut here ]------------
[ 68.700692] kernel BUG at fs/xfs/xfs_message.c:100!
[ 68.700742] invalid opcode: 0000 [#1] SMP
........
[ 68.701678] Pid: 4057, comm: mkfs.xfs Tainted: G O 3.9.0-rc2 #1
[ 68.701722] EIP: 0060:[<f94a7e8b>] EFLAGS: 00010282 CPU: 0
[ 68.701783] EIP is at assfail+0x2b/0x30 [xfs]
[ 68.701819] EAX: 00000056 EBX: f6ef28a0 ECX: 00000007 EDX: f57d22a4
[ 68.701852] ESI: 1c2fb000 EDI: 00000000 EBP: ea6b5d30 ESP: ea6b5d1c
[ 68.701895] DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
[ 68.701934] CR0: 8005003b CR2: 094f3ff4 CR3: 2bcb4000 CR4: 000006f0
[ 68.701970] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
[ 68.702011] DR6: ffff0ff0 DR7: 00000400
[ 68.702046] Process mkfs.xfs (pid: 4057, ti=ea6b4000 task=ea5799e0 task.ti=ea6b4000)
[ 68.702086] Stack:
[ 68.702124] 00000000 f9525c48 f951fa80 f951f96b 000005e4 ea6b5d7c f9494b34 c19b0ea2
[ 68.702445] 00000066 f3d6c620 c19b0ea2 00000000 e9a91458 00001000 00000000 00000000
[ 68.702868] 00000000 c15c7e89 00000000 1c2fb000 00000000 00000000 1c2fb000 00000080
[ 68.703192] Call Trace:
[ 68.703248] [<f9494b34>] xfs_vm_write_failed+0x74/0x1b0 [xfs]
[ 68.703441] [<c15c7e89>] ? printk+0x4d/0x4f
[ 68.703496] [<f9494d7d>] xfs_vm_write_begin+0x10d/0x170 [xfs]
[ 68.703535] [<c110a34c>] generic_file_buffered_write+0xdc/0x210
[ 68.703583] [<f949b669>] xfs_file_buffered_aio_write+0xf9/0x190 [xfs]
[ 68.703629] [<f949b7f3>] xfs_file_aio_write+0xf3/0x160 [xfs]
[ 68.703668] [<c115e504>] do_sync_write+0x94/0xd0
[ 68.703716] [<c115ed1f>] vfs_write+0x8f/0x160
[ 68.703753] [<c115e470>] ? wait_on_retry_sync_kiocb+0x50/0x50
[ 68.703794] [<c115f017>] sys_write+0x47/0x80
[ 68.703830] [<c15d860d>] sysenter_do_call+0x12/0x28
.............
[ 68.704203] EIP: [<f94a7e8b>] assfail+0x2b/0x30 [xfs] SS:ESP 0068:ea6b5d1c
[ 68.706615] ---[ end trace cdd9af4f4ecab42f ]---
[ 68.706687] Kernel panic - not syncing: Fatal exception
This patch fix the block_offset evaluation to clear the lower 12 bits as:
block_offset = pos >> PAGE_CACHE_SHIFT) << PAGE_CACHE_SHIFT
Hence, the ASSERTION should be correct because the from offset in a page is
evaluated to have the lower 12 bits only.
Thanks Dave Chinner for help finding and fixing this bug.
Reported-by: Michael L. Semon <mlsemon35@xxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Signed-off-by: Jie Liu <jeff.liu@xxxxxxxxxx>
---
fs/xfs/xfs_aops.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 5f707e5..f26a341 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1494,13 +1494,25 @@ xfs_vm_write_failed(
loff_t pos,
unsigned len)
{
- loff_t block_offset = pos & PAGE_MASK;
+ loff_t block_offset;
loff_t block_start;
loff_t block_end;
loff_t from = pos & (PAGE_CACHE_SIZE - 1);
loff_t to = from + len;
struct buffer_head *bh, *head;
+ /*
+ * The request pos offset might be 32 or 64 bit, this is all fine
+ * on 64-bit platform. However, for 64-bit pos request on 32-bit
+ * platform, the high 32-bit will be masked off if we evaluate the
+ * block_offset via (pos & PAGE_MASK) because the PAGE_MASK is
+ * 0xfffff000 as an unsigned long, hence the result is incorrect
+ * which could cause the following ASSERT failed in most cases.
+ * In order to avoid this, we can evaluate the block_offset with
+ * the lower 12-bit masked out and the ASSERT should be correct.
+ */
+ block_offset = (pos >> PAGE_CACHE_SHIFT) << PAGE_CACHE_SHIFT;
+
ASSERT(block_offset + from == pos);
head = page_buffers(page);
--
1.7.9.5
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