linux-kernel - Re: [PATCHv2] zram: do not waste zram_table

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 12 Sep 2022 22:07:08 -0400
From:   Brian Geffon <bgeffon@...gle.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nitin Gupta <ngupta@...are.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCHv2] zram: do not waste zram_table_entry flags bits

On Mon, Sep 12, 2022 at 9:54 PM Sergey Senozhatsky
<senozhatsky@...omium.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@...omium.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@...gle.com>