linux-kernel - Re: [PATCH v5 2/5] lib/test_bitmap: add tests for bitmap

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Date:   Thu, 28 Sep 2023 17:14:55 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, pcc@...gle.com,
        Andrey Konovalov <andreyknvl@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        eugenis@...gle.com, Syed Nayyar Waris <syednwaris@...il.com>,
        william.gray@...aro.org
Subject: Re: [PATCH v5 2/5] lib/test_bitmap: add tests for bitmap_{read,write}()

On Thu, Sep 28, 2023 at 4:43 PM Yury Norov <yury.norov@...il.com> wrote:
>
>
>
> On Thu, Sep 28, 2023, 10:20 AM Alexander Potapenko <glider@...gle.com> wrote:
>>
>> On Wed, Sep 27, 2023 at 9:51 AM David Laight <David.Laight@...lab.com> wrote:
>> >
>> > ...
>> > > Overall, unless allocating and initializing bitmaps with size
>> > > divisible by sizeof(long), most of bitmap.c is undefined behavior, so
>> > > I don't think it makes much sense to specifically test this case here
>> > > (given that we do not extend bitmap_equal() in the patch set).
>> >
>> > Bitmaps are arrays of unsigned long.
>> > Using any of the APIs on anything else is a bug.
>> > So it is always wrong to try to initialise 'a number of bytes'.
>> > The size used in the definition need not be a multiple of 8 (on 64bit)
>> > but the allocated data is always a multiple of 8.
>> >
>> > Any calls to the functions that have a cast of the bitmap
>> > parameter are likely to be buggy.
>> > And yes, there are loads of them, and many are buggy.
>>
>> I got rid of the casts in the bitmap test, but they remain in
>> mtecomp.c, where 16-, 32-, 64-byte buffers allocated by
>> kmem_cache_alloc() are treated as bitmaps:
>> https://lore.kernel.org/linux-arm-kernel/20230922080848.1261487-6-glider@google.com/T/#mdb0d636d2d357f8ffe6ac79cef1145df3440f659
>>
>> Having them allocated by bitmap_alloc() won't work, because on Android
>> bitmap_alloc() will allocate the buffers from the kmalloc-64 cache,
>> defeating the purpose of the compression.
>>
>> Would it be better to extend the bitmap.h API so that it is possible
>> to allocate from a kmem cache (which would in turn require
>> bitmap_kmem_cache_create() to ensure the alignment requirements)?
>
>
> So all that is wrong then. Bad on me, I'd spend more time looking into your driver code...
>
> We already have bitmap_(from,to)_u(64,32),
> And you can use them. For 16-bit you have to add helpers yourself. But it's not a rocket science.
>

So e.g. for compressing something into a 16-byte buffer using bitmaps
I'd need to:

1) Allocate the buffer: buf = kmem_cache_alloc(...)
2) Allocate the bitmap: bitmap = bitmap_alloc(16*8, ...)
3) Fill the bitmap: mte_compress_to_buf(..., bitmap, 16)
4) Copy the bitmap contents to the buffer: bitmap_to_arr64(buf, bitmap, 16*8)
5) Deallocate the bitmap: bitmap_free(bitmap)

instead of:

buf = kmem_cache_alloc(...)
mte_compress_to_buf(..., (unsigned long *)buf, 16)

, correct?

Given that the buffer contents are opaque and its size is aligned on 8
bytes, could it be possible to somehow adopt the `buf` pointer
instead?

> I'm AFK at the moment. I'll take a close look at your machinery at the weekend.

Thanks and take your time!