linux-kernel - Re: [PATCH] z3fold: use %z modifier for format string

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Date:   Fri, 25 Nov 2016 16:51:03 +0100
From:   Vitaly Wool <vitalywool@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Streetman <ddstreet@...e.org>,
        zhong jiang <zhongjiang@...wei.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] z3fold: use %z modifier for format string

On Fri, Nov 25, 2016 at 9:41 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Friday, November 25, 2016 8:38:25 AM CET Vitaly Wool wrote:
>> >> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> >> index e282ba073e77..66ac7a7dc934 100644
>> >> --- a/mm/z3fold.c
>> >> +++ b/mm/z3fold.c
>> >> @@ -884,7 +884,7 @@ static int __init init_z3fold(void)
>> >>  {
>> >>       /* Fail the initialization if z3fold header won't fit in one chunk */
>> >>       if (sizeof(struct z3fold_header) > ZHDR_SIZE_ALIGNED) {
>> >> -             pr_err("z3fold: z3fold_header size (%d) is bigger than "
>> >> +             pr_err("z3fold: z3fold_header size (%zd) is bigger than "
>> >>                       "the chunk size (%d), can't proceed\n",
>> >>                       sizeof(struct z3fold_header) , ZHDR_SIZE_ALIGNED);
>> >>               return -E2BIG;
>> >
>> > The embedded "z3fold: " prefix here should be removed
>> > as there's a pr_fmt that also adds it.
>> >
>> > The test looks like it should be a BUILD_BUG_ON rather
>> > than any runtime test too.
>>
>> It used to be BUILD_BUG_ON but we deliberately changed that because
>> sizeof(spinlock_t) gets bloated in debug builds, so it just won't
>> build with default CHUNK_SIZE.
>
> Could this be improved by making the CHUNK_SIZE bigger depending on
> the debug options?

I don't see how silently enforcing a suboptimal configuration is
better than failing the initialization (so that you can adjust
CHUNK_SIZE yourself). I can add something descriptive to
Documentation/vm/z3fold.txt for that matter.

> Alternatively, how about using a bit_spin_lock instead of raw_spin_lock?
> That would guarantee a fixed size for the lock and make z3fold_header
> always 24 bytes (on 32-bit architectures) or 40 bytes
> (on 64-bit architectures). You could even play some tricks with the
> first_num field to make it fit in the same word as the lock and make the
> structure fit into 32 bytes if you care about that.

That is interesting. Actually I can have that bit in page->private and
then I don't need to handle headless pages in a special way, that
sounds appealing. However, there is a warning about bit_spin_lock
performance penalty. Do you know how big it is?

Best regards,
   Vitaly