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

[JoonSoo Kim]

Hello, Vivek.

2012/12/7 Vivek Goyal <vgoyal@xxxxxxxxxx>:
> On Fri, Dec 07, 2012 at 10:16:55PM +0900, JoonSoo Kim wrote:
>> 2012/12/7 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
>> > On Fri,  7 Dec 2012 01:09:27 +0900
>> > Joonsoo Kim <js1304@xxxxxxxxx> wrote:
>> >
>> >> I'm not sure that "7/8: makes vmlist only for kexec" is fine.
>> >> Because it is related to userspace program.
>> >> As far as I know, makedumpfile use kexec's output information and it only
>> >> need first address of vmalloc layer. So my implementation reflect this
>> >> fact, but I'm not sure. And now, I don't fully test this patchset.
>> >> Basic operation work well, but I don't test kexec. So I send this
>> >> patchset with 'RFC'.
>> >
>> > Yes, this is irritating.  Perhaps Vivek or one of the other kexec
>> > people could take a look at this please - if would obviously be much
>> > better if we can avoid merging [patch 7/8] at all.
>>
>> I'm not sure, but I almost sure that [patch 7/8] have no problem.
>> In kexec.c, they write an address of vmlist and offset of vm_struct's
>> address field.
>> It imply that user for this information doesn't have any other
>> information about vm_struct,
>> and they can't use other field of vm_struct. They can use *only* address field.
>> So, remaining just one vm_struct for vmlist which represent first area
>> of vmalloc layer
>> may be safe.
>
> I browsed through makedumpfile source quickly. So yes it does look like
> that we look at first vmlist element ->addr field to figure out where
> vmalloc area is starting.
>
> Can we get the same information from this rb-tree of vmap_area? Is
> ->va_start field communication same information as vmlist was
> communicating? What's the difference between vmap_area_root and vmlist.

Thanks for comment.

Yes. vmap_area's va_start field represent same information as vm_struct's addr.
vmap_area_root is data structure for fast searching an area.
vmap_area_list is address sorted list, so we can use it like as vmlist.

There is a little difference vmap_area_list and vmlist.
vmlist is lack of information about some areas in vmalloc address space.
For example, vm_map_ram() allocate area in vmalloc address space,
but it doesn't make a link with vmlist. To provide full information
about vmalloc address space,
using vmap_area_list is more adequate.

> 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. :)

Thanks.
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