linux-kernel - Re: [PATCH 3/4] bitmap: Introduce bitmap

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Message-ID: <effcd39c-7f86-5f69-0663-4b81015cfc7c@wanadoo.fr>
Date:   Sun, 3 Jul 2022 22:41:27 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     agk@...hat.com, snitzer@...nel.org, dm-devel@...hat.com,
        vneethv@...ux.ibm.com, oberpar@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        almaz.alexandrovich@...agon-software.com, linux@...musvillemoes.dk,
        linux-s390@...r.kernel.org, ntfs3@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 3/4] bitmap: Introduce bitmap_size()

Le 03/07/2022 à 21:13, Yury Norov a écrit :
> On Sun, Jul 03, 2022 at 06:20:53PM +0300, Andy Shevchenko wrote:
>> On Sun, Jul 03, 2022 at 08:50:19AM +0200, Christophe JAILLET wrote:
>>> Le 02/07/2022 à 23:09, Yury Norov a écrit :
>>>> On Sat, Jul 02, 2022 at 08:29:36PM +0200, Christophe JAILLET wrote:
>>
>> ...
>>
>>>> This should be dropped, for sure, and kmalloc() at line 128 should be
>>>> replaced with bitmap_alloc().
>>>
>>> This kmalloc() is for a structure and a flexible array.
>>>
>>> You mean re-arranging the code to allocate the structure alone at first,
>>> then the bitmap?
> 
> We can change struct primes to:
>          struct primes {
>                  struct rcu_head rcu;
>                  unsigned long last, sz;
>                  unsigned long *primes;
>          };
> 
> And then either allocate twice:
>          new = kmalloc(sizeof(struct primes);
>          new->primes = bitmap_alloc(sz);
> 
> Or keep the same struct primes for all expansions, and just allocate
> new bitmap for ->primes when needed. This is what I meant.
> 
> This a bit deeper rework, but it addresses Andy's concern about excessive
> fragmentation. (Did anyone before complain? Is it measurable?)
> 
>> It's one way, but it will increase fragmentation of memory. The other one
>> as it seems to me is to name a new API properly, i.e. bitmap_size_to_bytes().
>>
>> In such case you won't need renames to begin with. And then would be able
>> to convert driver-by-driver in cases of duplicated code.
>>
>> I think that's what confused Yuri and I kinda agree that bitmap_size() should
>> return bits, and not bytes. Also argument for pure bitmap_size() would be
>> bitmap itself, but we have no way to detect the length of bitmap because we
>> are using POD and not a specific data structure for it.
> 
> bitmap_size_to_bytes() sounds better. How many places in the kernel
> do we have where we can't simply use bitmap_alloc(), and need this
> machinery? If this is the only one, I'd prefer to switch it to
> bitmap_alloc() instead.

I'll spot some places that would require a bitmap_size_to_bytes().

This way, we'll have some more information to decide if:
    - bitmap_size_to_bytes() makes sense or not
    - other helper functions are better suited
    - these places need some rework to use the existing API

CJ

> 
> Thanks,
> Yury
>