Linux 的换页机制是脏页机制,也就是写一个文件是通过写内存缓存页,然后标记脏位,在换页的时候一次性写回磁盘。而不是每次都写磁盘。
在打开文件的时候可以用 O_DIRECT | O_SYNC 来标记对脏页进行直写策略,换回同步。
但这里并不是对文件进行硬存级别的篡改,假设文件进了内存,然后我修改了这个文件内存中的部分内容,短时间再次访问这个文件会直接从内存缓存中拿到我们篡改过的内容而不是去 disk 上取。
structiov_iter { /* * Bit 0 is the read/write bit, set if we're writing. * Bit 1 is the BVEC_FLAG_NO_REF bit, set if type is a bvec and * the caller isn't expecting to drop a page reference when done. */ unsignedint type; size_t iov_offset; size_t count; union{ (···) structpipe_inode_info *pipe; }; (···) };
wget https://github.com/torvalds/linux/archive/refs/tags/v5.13-rc1.tar.gz make menuconfig cat /proc/cpuinfo| grep "cpu cores" | uniq # check cpu core counts make bzImage -j4
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kernel hacking ->compile-time checks and compiler opt -> compile the kernel with debug info -> Provide GDB scripts for kernel debugging -> Generate readable assembler code kernel hacking -> compile the kernel with frame pointers (没找到) kernel hacking -> Generic Kernel Debugging Instruments -> KGDB: kernel debugger -> KGDB: use kgdb over the serial console (NEW)
If fd_in refers to a pipe, then off_in must be NULL
If fd_in does not refer to a pipe and off_in is NULL, then bytes are read from fd_in starting from the file offset, and the file offset is adjusted appropriately.
/** * Create a pipe where all "bufs" on the pipe_inode_info ring have the * PIPE_BUF_FLAG_CAN_MERGE flag set. */ staticvoidprepare_pipe(int p[2]) { if (pipe(p)) abort();
/* fill the pipe completely; each pipe_buffer will now have the PIPE_BUF_FLAG_CAN_MERGE flag */ for (unsigned r = pipe_size; r > 0;) { unsigned n = r > sizeof(buffer) ? sizeof(buffer) : r; write(p[1], buffer, n); r -= n; }
/* drain the pipe, freeing all pipe_buffer instances (but leaving the flags initialized) */ for (unsigned r = pipe_size; r > 0;) { unsigned n = r > sizeof(buffer) ? sizeof(buffer) : r; read(p[0], buffer, n); r -= n; }
/* the pipe is now empty, and if somebody adds a new pipe_buffer without initializing its "flags", the buffer will be mergeable */ }
/* open the input file and validate the specified offset */ constint fd = open(path, O_RDONLY); // yes, read-only! :-) if (fd < 0) { perror("open failed"); return EXIT_FAILURE; }
structstatst; if (fstat(fd, &st)) { perror("stat failed"); return EXIT_FAILURE; }
if (offset > st.st_size) { fprintf(stderr, "Offset is not inside the file\n"); return EXIT_FAILURE; }
if (end_offset > st.st_size) { fprintf(stderr, "Sorry, cannot enlarge the file\n"); return EXIT_FAILURE; }
/* create the pipe with all flags initialized with PIPE_BUF_FLAG_CAN_MERGE */ int p[2]; prepare_pipe(p);
/* splice one byte from before the specified offset into the pipe; this will add a reference to the page cache, but since copy_page_to_iter_pipe() does not initialize the "flags", PIPE_BUF_FLAG_CAN_MERGE is still set */ --offset; ssize_t nbytes = splice(fd, &offset, p[1], NULL, 1, 0); if (nbytes < 0) { perror("splice failed"); return EXIT_FAILURE; } if (nbytes == 0) { fprintf(stderr, "short splice\n"); return EXIT_FAILURE; }
/* the following write will not create a new pipe_buffer, but will instead write into the page cache, because of the PIPE_BUF_FLAG_CAN_MERGE flag */ nbytes = write(p[1], data, data_size); if (nbytes < 0) { perror("write failed"); return EXIT_FAILURE; } if ((size_t)nbytes < data_size) { fprintf(stderr, "short write\n"); return EXIT_FAILURE; }