Messages in this thread | | | From | Alexander Potapenko <> | Date | Thu, 28 Sep 2023 16:19:56 +0200 | Subject | Re: [PATCH v5 2/5] lib/test_bitmap: add tests for bitmap_{read,write}() |
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On Wed, Sep 27, 2023 at 9:51 AM David Laight <David.Laight@aculab.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)?
> 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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