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


Thanks
Atsushi Kumagai


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