linux-kernel - Re: [PATCH 2/3] z3fold: don't fail kernel build if z3fold

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Date:   Fri, 25 Nov 2016 17:25:42 +0100
From:   Vitaly Wool <vitalywool@...il.com>
To:     Dan Streetman <ddstreet@...e.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 2/3] z3fold: don't fail kernel build if z3fold_header is
 too big

On Fri, Nov 25, 2016 at 4:59 PM, Dan Streetman <ddstreet@...e.org> wrote:
> On Tue, Nov 15, 2016 at 11:00 AM, Vitaly Wool <vitalywool@...il.com> wrote:
>> Currently the whole kernel build will be stopped if the size of
>> struct z3fold_header is greater than the size of one chunk, which
>> is 64 bytes by default. This may stand in the way of automated
>> test/debug builds so let's remove that and just fail the z3fold
>> initialization in such case instead.
>>
>> Signed-off-by: Vitaly Wool <vitalywool@...il.com>
>> ---
>>  mm/z3fold.c | 11 ++++++++---
>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index 7ad70fa..ffd9353 100644
>> --- a/mm/z3fold.c
>> +++ b/mm/z3fold.c
>> @@ -870,10 +870,15 @@ MODULE_ALIAS("zpool-z3fold");
>>
>>  static int __init init_z3fold(void)
>>  {
>> -       /* Make sure the z3fold header will fit in one chunk */
>> -       BUILD_BUG_ON(sizeof(struct z3fold_header) > ZHDR_SIZE_ALIGNED);
>
> Nak.  this is the wrong way to handle this.  The build bug is there to
> indicate to you that your patch makes the header too large, not as a
> runtime check to disable everything.

Okay, let's agree to drop it.

> The right way to handle it is to change the hardcoded assumption that
> the header fits into a single chunk; e.g.:
>
> #define ZHDR_SIZE_ALIGNED round_up(sizeof(struct z3fold_header), CHUNK_SIZE)
> #define ZHDR_CHUNKS (ZHDR_SIZE_ALIGNED >> CHUNK_SHIFT)
>
> then use ZHDR_CHUNKS in all places where it's currently assumed the
> header is 1 chunk, e.g. in num_free_chunks:
>
>   if (zhdr->middle_chunks != 0) {
>     int nfree_before = zhdr->first_chunks ?
> -      0 : zhdr->start_middle - 1;
> +      0 : zhdr->start_middle - ZHDR_CHUNKS;
>
> after changing all needed places like that, the build bug isn't needed
> anymore (unless we want to make sure the header isn't larger than some
> arbitrary number N chunks)

That sounds overly complicated to me. I would rather use bit_spin_lock
as Arnd suggested. What would you say?

~vitaly