Re: [RFC] Tagging of vmalloc pages for supporting the pmalloc allocator

[Jerome Glisse]

On Mon, Aug 07, 2017 at 02:26:21PM +0300, Igor Stoppa wrote:
> On 04/08/17 11:12, Michal Hocko wrote:
> > On Fri 04-08-17 11:02:46, Igor Stoppa wrote:
> 
> [...]
> 
> >> struct page {
> >>   /* First double word block */
> >>   unsigned long flags;		/* Atomic flags, some possibly
> >> 				 * updated asynchronously */
> >> union {
> >> 	struct address_space *mapping;	/* If low bit clear, points to
> >> 					 * inode address_space, or NULL.
> >> 					 * If page mapped as anonymous
> >> 					 * memory, low bit is set, and
> >> 					 * it points to anon_vma object:
> >> 					 * see PAGE_MAPPING_ANON below.
> >> 					 */
> >> ...
> >> }
> >>
> >> mapping seems to be used exclusively in 2 ways, based on the value of
> >> its lower bit.
> > 
> > Not really. The above applies to LRU pages. Please note that Slab pages
> > use s_mem and huge pages use compound_mapcount. If vmalloc pages are
> > using none of those already you can add a new field there.
> 
> Yes, both from reading the code and some experimentation, it seems that
> vmalloc is not using either field.
> 
> I'll add a vm_area field as you advised.
> 
> Is this something I could send as standalone patch?

Note that vmalloc() is not the only thing that use vmalloc address
space. There is also vmap() and i know one set of drivers that use
vmap() and also use the mapping field of struct page namely GPU
drivers.

So like i said previously i would store a flag inside vm_struct to
know if page you are looking at are pmalloc or not. Again do you
need to store something per page ? Would storing it per vm_struct
not be enough ?

Cheers,
Jérôme