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

[Michal Hocko]

On Thu 03-08-17 15:20:31, Igor Stoppa wrote:
> On 03/08/17 14:48, Michal Hocko wrote:
> > On Thu 03-08-17 13:11:45, Igor Stoppa wrote:
> >> On 02/08/17 20:08, Jerome Glisse wrote:
> >>> On Wed, Aug 02, 2017 at 06:14:28PM +0300, Igor Stoppa wrote:
> 
> [...]
> 
> >>>> from include/linux/mm_types.h:
> >>>>
> >>>> struct page {
> >>>> ...
> >>>>   union {
> >>>>     unsigned long private;		/* Mapping-private opaque data:
> >>>> 				 	 * usually used for buffer_heads
> >>>> 					 * if PagePrivate set; used for
> >>>> 					 * swp_entry_t if PageSwapCache;
> >>>> 					 * indicates order in the buddy
> >>>> 					 * system if PG_buddy is set.
> >>>> 					 */
> 
> [...]
> 
> >> If the "Mapping-private" was dropped or somehow connected exclusively to
> >> the cases listed in the comment, then I think it would be more clear
> >> that the comment needs to be intended as related to mapping in certain
> >> cases only.
> >> But it is otherwise ok to use the "private" field for whatever purpose
> >> it might be suitable, as long as it is not already in use.
> > 
> > I would recommend adding a new field into the enum...
> 
> s/enum/union/ ?
> 
> If not, I am not sure what is the enum that you are talking about.

yeah, fat fingers on my side

> 
> [...]
> 
> >> But, to reply more specifically to your advice, yes, I think I could add
> >> a flag to vm_struct and then retrieve its value, for the address being
> >> processed, by passing through find_vm_area().
> > 
> > ... and you can store vm_struct pointer to the struct page there 
> 
> "there" as in the new field of the union?
> btw, what would be a meaningful name, since "private" is already taken?
> 
> For simplicity, I'll use, for now, "private2"

why not explicit vm_area?

> > and you> won't need to do the slow find_vm_area. I haven't checked
> very closely
> > but this should be possible in principle. I guess other callers might
> > benefit from this as well.
> 
> I am confused about this: if "private2" is a pointer, but when I get an
> address, I do not even know if the address represents a valid pmalloc
> page, how can i know when it's ok to dereference "private2"?

because you can make all pages which back vmalloc mappings have vm_area
pointer set.

-- 
Michal Hocko
SUSE Labs