Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree

["HAGIO KAZUHITO(萩尾 一仁)" <k-hagio-ab@nec.com>]

On 2023/09/07 18:58, Baoquan He wrote:
> On 09/07/23 at 11:39am, Uladzislau Rezki wrote:
>> On Thu, Sep 07, 2023 at 10:17:39AM +0800, Baoquan He wrote:
>>> Add Kazu and Lianbo to CC, and kexec mailing list
>>>
>>> On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
>>>> Store allocated objects in a separate nodes. A va->va_start
>>>> address is converted into a correct node where it should
>>>> be placed and resided. An addr_to_node() function is used
>>>> to do a proper address conversion to determine a node that
>>>> contains a VA.
>>>>
>>>> Such approach balances VAs across nodes as a result an access
>>>> becomes scalable. Number of nodes in a system depends on number
>>>> of CPUs divided by two. The density factor in this case is 1/2.
>>>>
>>>> Please note:
>>>>
>>>> 1. As of now allocated VAs are bound to a node-0. It means the
>>>>     patch does not give any difference comparing with a current
>>>>     behavior;
>>>>
>>>> 2. The global vmap_area_lock, vmap_area_root are removed as there
>>>>     is no need in it anymore. The vmap_area_list is still kept and
>>>>     is _empty_. It is exported for a kexec only;
>>>
>>> I haven't taken a test, while accessing all nodes' busy tree to get
>>> va of the lowest address could severely impact kcore reading efficiency
>>> on system with many vmap nodes. People doing live debugging via
>>> /proc/kcore will get a little surprise.
>>>
>>>
>>> Empty vmap_area_list will break makedumpfile utility, Crash utility
>>> could be impactd too. I checked makedumpfile code, it relys on
>>> vmap_area_list to deduce the vmalloc_start value.
>>>
>> It is left part and i hope i fix it in v3. The problem here is
>> we can not give an opportunity to access to vmap internals from
>> outside. This is just not correct, i.e. you are not allowed to
>> access the list directly.
> 
> Right. Thanks for the fix in v3, that is a relief of makedumpfile and
> crash.
> 
> Hi Kazu,
> 
> Meanwhile, I am thinking if we should evaluate the necessity of
> vmap_area_list in makedumpfile and Crash. In makedumpfile, we just use
> vmap_area_list to deduce VMALLOC_START. Wondering if we can export
> VMALLOC_START directly. Surely, the lowest va->va_start in vmap_area_list
> is a tighter low boundary of vmalloc area and can reduce unnecessary
> scanning below the lowest va. Not sure if this is the reason people
> decided to export vmap_area_list.

The kernel commit acd99dbf5402 introduced the original vmlist entry to 
vmcoreinfo, but there is no information about why it did not export 
VMALLOC_START directly.

If VMALLOC_START is exported directly to vmcoreinfo, I think it would be 
enough for makedumpfile.

Thanks,
Kazu