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Date:   Tue, 13 Sep 2022 10:54:34 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Brian Geffon <bgeffon@google.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nitin Gupta <ngupta@vflare.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCHv2] zram: do not waste zram_table_entry flags bits
Message-ID: <Yx/i2nQ/XVG334ag@google.com>
References: <20220912152744.527438-1-senozhatsky@chromium.org>
 <CADyq12z8ijE__cgxdMssoE6Ooz4zdazsZ-TeipLTNqqis2vSzA@mail.gmail.com>
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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.