Re: [PATCH v3] mm/page_owner: use kvmalloc instead of kmalloc

[Miles Chen]

On Tue, 2018-10-30 at 07:06 +0100, Michal Hocko wrote:
> On Tue 30-10-18 09:29:10, Miles Chen wrote:
> > On Mon, 2018-10-29 at 09:17 +0100, Michal Hocko wrote:
> > > On Mon 29-10-18 09:07:08, Michal Hocko wrote:
> > > [...]
> > > > Besides that, the following doesn't make much sense to me. It simply
> > > > makes no sense to use vmalloc for sub page allocation regardless of
> > > > HIGHMEM.
> > > 
> > > OK, it is still early morning here. Now I get the point of the patch.
> > > You just want to (ab)use highmeme for smaller requests. I do not like
> > > this, to be honest. It causes an internal fragmentation and more
> > > importantly the VMALLOC space on 32b where HIGHMEM is enabled (do we
> > > have any 64b with HIGHMEM btw?) is quite small to be wasted like that.
> > > 
> > thanks for your comment. It looks like that using vmalloc fallback for
> > sub page allocation is not good here.
> > 
> > Your comment gave another idea:
> > 
> > 1. force kbuf to PAGE_SIZE
> > 2. allocate a page by alloc_page(GFP_KERNEL | __GFP_HIGHMEM); so we can
> > get a highmem page if possible
> > 3. use kmap/kunmap pair to create mapping for this page. No vmalloc
> > space is used.
> > 4. do not change kvmalloc logic.
> 
> If you mean for this particular situation then is this really worth
> it? I mean this is a short term allocation for root only so you do not
> have to worry about low mem depletion.

The 1...3 are applied to print_page_owner(), not in kmalloc() or
kvmalloc() logic. 


It's a real problem when using page_owner.
I found this issue recently: I'm not able to read page_owner information
during a overnight test. (error: read failed: Out of memory). I replace
kmalloc() with vmalloc() and it worked well.

> 
> If you are thiking in more generic terms to allow kmalloc to use highmem
> then I am not really sure this will work out.

I'm thinking about modify print_page_owner().