linux-kernel - Re: [PATCH/RFC] z3fold: use per-page read/write lock

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:   Sun, 6 Nov 2016 10:42:49 +0100
From:   Vitaly Wool <vitalywool@...il.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Dan Streetman <ddstreet@...e.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH/RFC] z3fold: use per-page read/write lock

On Sun, Nov 6, 2016 at 12:38 AM, Andi Kleen <andi@...stfloor.org> wrote:
> Vitaly Wool <vitalywool@...il.com> writes:
>
>> Most of z3fold operations are in-page, such as modifying z3fold
>> page header or moving z3fold objects within a page. Taking
>> per-pool spinlock to protect per-page objects is therefore
>> suboptimal, and the idea of having a per-page spinlock (or rwlock)
>> has been around for some time. However, adding one directly to the
>> z3fold header makes the latter quite big on some systems so that
>> it won't fit in a signle chunk.
>
>> +     atomic_t page_lock;
>
> This doesnt make much sense. A standard spinlock is not bigger
> than 4 bytes either. Also reinventing locks is usually a bad
> idea: they are tricky to get right, you have no debugging support,
> hard to analyze, etc.

I understand the reinvention part but you're not quite accurate here
with the numbers.

E. g. on x86_64:
(gdb) p sizeof(rwlock_t)
$1 = 8

I believe a DIY lock is justified here, since the variant with
rwlock_t actually caused complaints from kbuild test robot building
the previous version of this patch [1] with gcc-6.0 for x86_64:

 In file included from arch/x86/include/asm/atomic.h:4:0,
                    from include/linux/atomic.h:4,
                    from mm/z3fold.c:25:
   mm/z3fold.c: In function 'init_z3fold':
>> include/linux/compiler.h:518:38: error: call to '__compiletime_assert_808' declared with attribute error: BUILD_BUG_ON failed: sizeof(struct z3fold_header) > ZHDR_SIZE_ALIGNED

~vitaly

[1] https://patchwork.kernel.org/patch/9384871/