On Fri 19-01-18 08:09:08, Petr Tesarik wrote: [...] > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 67f2e3c38939..7522a6987595 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -1166,8 +1166,16 @@ extern unsigned long usemap_size(void); > > /* > * We use the lower bits of the mem_map pointer to store > - * a little bit of information. There should be at least > - * 3 bits here due to 32-bit alignment. > + * a little bit of information. The pointer is calculated > + * as mem_map - section_nr_to_pfn(pnum). The result is > + * aligned to the minimum alignment of the two values: > + * 1. All mem_map arrays are page-aligned. > + * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT > + * lowest bits. PFN_SECTION_SHIFT is arch-specific > + * (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the > + * worst combination is powerpc with 256k pages, > + * which results in PFN_SECTION_SHIFT equal 6. > + * To sum it up, at least 6 bits are available. > */
This is _much_ better indeed. Do you think we can go one step further and add BUG_ON into the sparse code to guarantee that every mmemap is indeed aligned properly so that SECTION_MAP_LAST_BIT-1 bits are never used? Thanks! > #define SECTION_MARKED_PRESENT (1UL<<0) > #define SECTION_HAS_MEM_MAP (1UL<<1) > -- > 2.13.6 -- Michal Hocko SUSE Labs
