Re: [PATCHv2] zram: do not waste zram_table_entry flags bits

[Sergey Senozhatsky]

On (22/09/12 11:39), Brian Geffon wrote:
> > zram_table_entry::flags stores object size in the lower bits and
> > zram pageflags in the upper bits. However, for some reason, we
> > use 24 lower bits, while maximum zram object size is PAGE_SIZE,
> > which requires PAGE_SHIFT bits (up to 16 on arm64). This wastes
> > 24 - PAGE_SHIFT bits that we can use for additional zram pageflags
> > instead.
> >
> > Also add a BUILD_BUG_ON() to alert us should we run out of bits
> > in zram_table_entry::flags.
> >
> > Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> > ---
> >  drivers/block/zram/zram_drv.c |  2 ++
> >  drivers/block/zram/zram_drv.h | 15 +++++++--------
> >  2 files changed, 9 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index f3948abce2f7..07913bcdb5c2 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -2449,6 +2449,8 @@ static int __init zram_init(void)
> >  {
> >         int ret;
> >
> > +       BUILD_BUG_ON(__NR_ZRAM_PAGEFLAGS > BITS_PER_LONG);
> 
> Should this be >= BITS_PER_LONG?

__NR_ZRAM_PAGEFLAGS == BITS_PER_LONG == 64 (e.g. on 64 bit host)
means that the last valid zram pageflag (and __NR_ZRAM_PAGEFLAGS
is not a valid pageflag) is __NR_ZRAM_PAGEFLAGS - 1, which is 63
and which is a valid BIT() offset for u64.

So __NR_ZRAM_PAGEFLAGS == BITS_PER_LONG should be a valid case.