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SubjectRe: [RFC PATCH 0/8] remove vm_struct list management
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Hello, Atsushi.

2012/12/12 Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>:
> Hello,
>
> On Tue, 11 Dec 2012 17:17:05 -0500 (EST)
> Dave Anderson <anderson@redhat.com> wrote:
>
>>
>>
>> ----- Original Message -----
>> > On Mon, Dec 10, 2012 at 11:40:47PM +0900, JoonSoo Kim wrote:
>> >
>> > [..]
>> > > > So without knowing details of both the data structures, I think if vmlist
>> > > > is going away, then user space tools should be able to traverse vmap_area_root
>> > > > rb tree. I am assuming it is sorted using ->addr field and we should be
>> > > > able to get vmalloc area start from there. It will just be a matter of
>> > > > exporting right fields to user space (instead of vmlist).
>> > >
>> > > There is address sorted list of vmap_area, vmap_area_list.
>> > > So we can use it for traversing vmalloc areas if it is necessary.
>> > > But, as I mentioned before, kexec write *just* address of vmlist and
>> > > offset of vm_struct's address field. It imply that they don't traverse vmlist,
>> > > because they didn't write vm_struct's next field which is needed for traversing.
>> > > Without vm_struct's next field, they have no method for traversing.
>> > > So, IMHO, assigning dummy vm_struct to vmlist which is implemented by [7/8] is
>> > > a safe way to maintain a compatibility of userspace tool. :)
>> >
>> > Actually the design of "makedumpfile" and "crash" tool is that they know
>> > about kernel data structures and they adopt to changes. So for major
>> > changes they keep track of kernel version numbers and if access the
>> > data structures accordingly.
>> >
>> > Currently we access first element of vmlist to determine start of vmalloc
>> > address. True we don't have to traverse the list.
>> >
>> > But as you mentioned we should be able to get same information by
>> > traversing to left most element of vmap_area_list rb tree. So I think
>> > instead of trying to retain vmlist first element just for backward
>> > compatibility, I will rather prefer get rid of that code completely
>> > from kernel and let user space tool traverse rbtree. Just export
>> > minimum needed info for traversal in user space.
>>
>> There's no need to traverse the rbtree. There is a vmap_area_list
>> linked list of vmap_area structures that is also sorted by virtual
>> address.
>>
>> All that makedumpfile would have to do is to access the first vmap_area
>> in the vmap_area_list -- as opposed to the way that it does now, which is
>> by accessing the first vm_struct in the to-be-obsoleted vmlist list.
>>
>> So it seems silly to keep the dummy "vmlist" around.
>
> I think so, I will modify makedumpfile to get the start address of vmalloc
> with vmap_area_list if the related symbols are provided as VMCOREINFO like
> vmlist.
> BTW, have we to consider other tools ?
> If it is clear, I think we can get rid of the dummy vmlist.

Good!
In next spin, I will remove dummy vmlist and export vmap_area_list symbol
for makedumpfile.

I don't know any other tools.
If anyone knows it, please let me know.

Thanks! Atsushi, Dave and Vivek.


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