Re: [Crash-utility] enhance bt command
On Mon, 2008-03-03 at 15:35 -0500, Dave Anderson wrote:
<snip>
>>>
>>> so we know the object address, which slab it is in, and the offset,
>>> (thus can derive the raw value), all in one shot?
>>>
>> You've done a fine bit of debugging your issue...
>>
>> But I think that's a bit of overkill for each address reference.
>>
>> To do it right it would have to walk slab chains to gather all of the
>> information needed by the "kmem -S" output, which can be extremely
>> time-consuming, and potentially error-prone if the slab chain is
>> corrupt or being modified while running on a live system.
>
> full agree. realized now.
>
> then here is my question, how "kmem -s <addr>" find out which slab
this
> address belong to? by only looking at the page?
The kmem_cache and containing slab addresses are stashed in unused
fields of the page structure of the object's virtual address.
ic. good to know this.
>
> then here is my revised suggestion. can we split this into 2 steps?
>
> (1) rd -S show [raw address: cache name]
>
> (this is a great to have since do rd 2 times, one with -S and another
> without -S is tedious.)
But in your case, for example, I'm presuming you've done a "bt -f",
and simply would like a symbols/slab translation of a chunk of memory
making up one of the stack frames, so you do a subsequent "rd -S" of
that memory area. Doesn't seem that tedious to me...
have to agree.
>
> (2) and kmem -s <address>
>
> show which slab it is in and optionally (with a new option like -O) show
> the object?
Not following -- "kmem -s <address>" does show which slab it's in.
i emphasize on "and optionally...", ;)
And if you're asking whether the object can be dumped as the
structure
type it is (?), well, how would it be possible from the crash utility's
viewpoint to even know what type of data structure it is?
no. what i mean is kmem -s <address> does not give me the object address
crash> kmem -s 000001007877c8d4
CACHE NAME OBJSIZE ALLOCATED TOTAL
SLABS SSIZE
10037ffc080 dentry_cache 240 9429 10560
660 4k
SLAB MEMORY TOTAL ALLOCATED FREE
1007877c040 1007877c088 16 9 7
FREE / [ALLOCATED]
[1007877c8d4]
i still have to use kmem -S to find out 1007877c808 is the object that
contain this address. nice if crash can do this for me.
If you want to look at all of the objects in a slab represented
as data structures, you're going to have supply the knowledge of
what data structure they are. It's simple enough, just do a "kmem -S"
into a file, delete everything except the object addresses that you're
interested in, insert "struct whatever" in front of each address, save
this is exactly what i did when i have to do work like this by using
gawk, tr, and grep.
the file -- and run it as crash input file.
how to do this? i know crash -i can run a file at beginning. but how to
run command in a file at any moment?
>
> (this is nice to have).
>
> thanks.
Look, you can have it do anything you want...
However, keep in mind that there are 4 diffenent kmem slab paradigms
supported (the "original", the first verion with per-cpu objects, the
second version with per-cpu objects, and now the completely new CONFIG_SLUB
subsystem). All of them would have to be supported. And the code that does
the object dumping is somewhat replicated and embedded in the "kmem -S"
support
code, so it's not a simple re-use of an existing function, which I've done
to get the simple "rd -S" output.
ic.
Dave
--
Ming Zhang
@#$%^ purging memory... (*!%
http://blackmagic02881.wordpress.com/
http://www.linkedin.com/in/blackmagic02881
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