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

[Brian Geffon <bgeffon@google.com>]

On Mon, Sep 12, 2022 at 9:54 PM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> 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@chromium.org>
> > > ---
> > >  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.

I apologize, you are correct.

Reviewed-by: Brian Geffon <bgeffon@google.com>