gistfile1.cpp

#define __ALIGNUP_MASK(x,mask)      	(((x)+(mask))&~(mask))
#define ALIGNUP_TO_POWEROF2BOUNDARY(x, a)	__ALIGNUP_MASK(x, (typeof(x))(a)-1) 

int TouchFilePages(int fd, uint64_t offset, size_t len)
{
        void *fileData;
        static const size_t pageSize = getpagesize();
        uint64_t upto, end;

        if (len == 0)
        {
                struct stat64 s;

                if (fstat64(fd, &s) == -1)
                        return -1;
                        
                end  = s.st_size;
        }
        else
                end = offset + len;

        upto    = Min<uint64_t>(end, ALIGNUP_TO_POWEROF2BOUNDARY(end, pageSize));
        offset  = ALIGNDOWN_TO_POWEROF2BOUNDARY(offset, pageSize);
        len     = upto - offset;

        const size_t pagesCnt = ((upto - offset) + (pageSize - 1)) / pageSize;

        fileData = mmap(NULL, pagesCnt * pageSize, PROT_READ, MAP_SHARED, fd, offset);
        if (fileData == MAP_FAILED)
                return -1;
        else
        {
                uint8_t *const desc = (uint8_t *)malloc(pagesCnt * sizeof(uint8_t));
                size_t inCoreCnt = 0;
                char page[pageSize];
                uint64_t it = 0;
                const uint64_t contentEnd = end - offset;
                const uint32_t lastPageIndex = pagesCnt - 1;

                if (mincore(fileData, len, desc) == -1)
                {
                        free(desc);
                        (void)munmap(fileData, len);
                        return -1;
                }

                (void)madvise(fileData, len, MADV_SEQUENTIAL);
                for (uint32_t i = 0; i != lastPageIndex; ++i, it+=pageSize)
                {
                        if (desc[i]&1)
                                ++inCoreCnt;
                        else
                                (void)memcpy(page, (char *)fileData + it, pageSize);
                }

                if (desc[lastPageIndex]&1)
                        ++inCoreCnt;
                else
                        (void)memcpy(page, (char *)fileData + it, contentEnd - it);

                free(desc);
                (void)inCoreCnt; // You may want to report this somewhere
        }
        (void)munmap(fileData, len);
        return 0;
}