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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 28 Sep 2023 16:19:56 +0200
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Subject: Re: [PATCH v5 2/5] lib/test_bitmap: add tests for bitmap_{read,write}()
To:     David Laight <David.Laight@aculab.com>
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On Wed, Sep 27, 2023 at 9:51=E2=80=AFAM David Laight <David.Laight@aculab.c=
om> 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@go=
ogle.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)?

> On LE you mostly get away with shorter memory allocations.
> But still get errors when trying to do locked operations
> on misaligned addresses.
>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1=
 1PT, UK
> Registration No: 1397386 (Wales)


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