Re: [Libguestfs] [PATCH nbdkit v3] common: Move shared bitmap code to a common library.

The cow and cache filters both use a bitmap mapping virtual disk blocks to status stored in the bitmap. The implementation of the bitmaps is very similar because one was derived from the other when the filters were implemented. The main difference is the cow filter uses a simple bitmap (one bit per block), whereas the cache filter uses two bits per block. This commit abstracts the bitmap structure into a common library. The block size and bits per block are configurable. This commit is refactoring, except: - it cures a memory leak (bitmap was not being freed), - in the cache filter we now mask out existing bits when updating the bitmap. --- + +/* This is the bitmap structure. */ +struct bitmap { + unsigned blksize; /* Block size. */ + unsigned bpb; /* Bits per block (1, 2, 4, 8 only). */ + /* bpb = 1 << bitshift ibpb = 8/bpb + 1 0 8 + 2 1 4 + 4 2 2 + 8 3 1 + */ + unsigned bitshift, ibpb;

+ + uint8_t *bitmap; /* The bitmap. */ + size_t size; /* Size of bitmap in bytes. */ +};

+/* This macro calculates the byte offset in the bitmap and which + * bit/mask we are addressing within that byte. + * + * bpb blk_offset blk_bit mask + * 1 blk >> 3 0,1,2,...,7 any single bit + * 2 blk >> 2 0, 2, 4 or 6 0x3, 0xc, 0x30 or 0xc0 + * 4 blk >> 1 0 or 4 0xf, 0xf0 + * 8 blk >> 0 always 0 always 0xff + */ +#define BITMAP_OFFSET_BIT_MASK(bm, blk) \ + uint64_t blk_offset = (blk) >> (3 - (bm)->bitshift); \ + unsigned blk_bit = (bm)->bpb * ((blk) & ((bm)->ibpb - 1)); \ + unsigned mask = ((1 << (bm)->bpb) - 1) << blk_bit