On (22/09/12 10:20), Brian Geffon wrote: > > /* > > - * The lower ZRAM_FLAG_SHIFT bits of table.flags is for > > - * object size (excluding header), the higher bits is for > > - * zram_pageflags. > > - * > > - * zram is mainly used for memory efficiency so we want to keep memory > > - * footprint small so we can squeeze size and flags into a field. > > + * ZRAM is mainly used for memory efficiency so we want to keep memory > > + * footprint small and thus squeeze size and flags into a flags member. > > * The lower ZRAM_FLAG_SHIFT bits is for object size (excluding header), > > - * the higher bits is for zram_pageflags. > > + * which cannot be larger than PAGE_SIZE (requiring PAGE_SHIFT bits), > > + * the higher bits are for zram_pageflags. > > */ > > -#define ZRAM_FLAG_SHIFT 24 > > +#define ZRAM_FLAG_SHIFT (PAGE_SHIFT + 1) > > Why not just hard code 16 with an explanation that it cannot be > increased further using the analysis you did in the other thread? It's > going to be tricky to reason about how many free flag bits actually > remain with PAGE_SHIFT across all architectures, especially given we > have no architecture specific flags. Well, zram should not make any assumptions on arch code. How do we know that PAGE_SHIFT 16 is the max value we will ever have? Some arch can come around someday and use PAGE_SHIFT say, 18, and we won't be aware of it (using hardcoded value of 16) until someone hits a really hard to debug problem in zram.
