Re: [RFC 3/6] mm: support madvise(MADV_FREE)

[Minchan Kim <minchan@kernel.org>]

On Fri, Mar 14, 2014 at 03:33:11PM +0200, Kirill A. Shutemov wrote:
> On Fri, Mar 14, 2014 at 03:37:47PM +0900, Minchan Kim wrote:
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index c1b7414c7bef..9b048cabce27 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -933,10 +933,16 @@ void page_address_init(void);
> >   * Please note that, confusingly, "page_mapping" refers to the inode
> >   * address_space which maps the page from disk; whereas "page_mapped"
> >   * refers to user virtual address space into which the page is mapped.
> > + *
> > + * PAGE_MAPPING_LZFREE bit is set along with PAGE_MAPPING_ANON bit
> > + * and then page->mapping points to an anon_vma. This flag is used
> > + * for lazy freeing the page instead of swap.
> >   */
> >  #define PAGE_MAPPING_ANON	1
> >  #define PAGE_MAPPING_KSM	2
> > -#define PAGE_MAPPING_FLAGS	(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM)
> > +#define PAGE_MAPPING_LZFREE	4
> > +#define PAGE_MAPPING_FLAGS	(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM | \
> > +				 PAGE_MAPPING_LZFREE)
> 
> Is it safe to use third bit in pointer everywhere?

I overlooked ARCH_SLAB_MINALIGN which is 8 byte for most arch but
surely some of arch would have less than it(ex, 4 byte).

Alternative is PG_private or PG_private2. That flags is used for
file pages mostly while zsmalloc uses it but it should not LRU page.
Other thing in mm/ is memory-hotplug but I guess we don't need to set
PG_private because I couldn't find PagePrivate for that.

So, I think using that flag for anon page has no problem if we tweak
page_has_private works with only !PageAnon.

> 
> -- 
>  Kirill A. Shutemov
> 
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