Messages in this thread |  | | | Date | Tue, 28 Jun 2005 20:54:47 +0200 | | From | Samuel Thibault <> | | Subject | Re: wrong madvise(MADV_DONTNEED) semantic |
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Andy Isaacson, le Tue 28 Jun 2005 11:16:20 -0700, a écrit : > If your interpretation of the problem is correct, then it should be > trivial to write a test program demonstrating the problem. Did you > write the simple test program and run it?
I indeed didn't, trusting both the man page, the source code comments, and my knowledge of zap_page_range().
> MADV_DONTNEED > Do not expect access in the near future. (For the time > being, the application is finished with the given range, > so the kernel can free resources associated with it.) > Subsequent accesses of pages in this range will succeed, > but will result either in reloading of the memory contents > from the underlying mapped file (see mmap) or > zero-fill-on-demand pages for mappings without an > underlying file. > > You seem to think that "reloading ... from the underlying mapped file" > means that changes are lost, but that's not implied.
I didn't say anything precise. What mostly feared me was the "zero-fill-on-demand pages for mappings without an underlying file."
> Below is the test program I used.
It does indeed work, but this is no proof. It your testcase it does indeed work, since the page still remains in the page cache (it's a shared mapping of the file). But now try this one. It uses private mappings, and fails as expected, getting 0 in the ANONYMOUS case, and the original file value in the file mapping case.
Regards, Samuel Thibault
#include <stdio.h> #include <stdlib.h> #include <stdarg.h> #include <string.h> #include <errno.h>
#include <signal.h> #include <unistd.h> #include <sys/mman.h> #include <fcntl.h> #include <sys/stat.h>
typedef unsigned int u32;
void die(char *fmt, ...) { va_list ap; va_start(ap, fmt); vfprintf(stderr, fmt, ap); va_end(ap); exit(1); } int check_cookie(char *file, u32 cookie) { u32 buf; int fd; if((fd = open(file, O_RDONLY, 0)) == -1) die("%s: %s\n", file, strerror(errno)); if(read(fd, &buf, sizeof(buf)) == -1) die("read: %s\n", strerror(errno)); close(fd); return buf == cookie; } int dotest(char *file, u32 cookie, int do_anonymous, int do_msync, int do_madvise) { void *p; int fd, len = 16 * 1024, prot = PROT_READ|PROT_WRITE; u32 newcookie; if (!do_anonymous) { if((fd = open(file, O_RDWR|O_CREAT, 0666)) == -1) die("%s: %s\n", file, strerror(errno)); if(ftruncate(fd, len) == -1) die("ftruncate: %s\n", strerror(errno)); if(write(fd, "", 1) == -1) die("write: %s\n", strerror(errno)); } if((p = mmap(0, len, prot, (do_anonymous ? MAP_ANONYMOUS : 0) | MAP_PRIVATE, do_anonymous ? -1 : fd, 0)) == MAP_FAILED) die("mmap: %s\n", strerror(errno)); *(u32 *)p = cookie;
if(do_msync) if(msync(p, len, MS_SYNC) == -1) die("msync: %s\n", strerror(errno)); if(do_madvise) if(madvise(p, len, MADV_DONTNEED) == -1) die("madvise: %s\n", strerror(errno)); newcookie = *(u32 *)p;
printf("c = %08x msync: %s madvise: %s %s\n", cookie, do_msync ? "YES" : " NO", do_madvise ? "YES" : " NO", newcookie == cookie ? "ok" : "FAILED"); } int main(int argc, char **argv) { if(argc != 2) die("usage: %s file\n", argv[0]); lrand48(); dotest(argv[1], lrand48(), 0, 0, 0); dotest(argv[1], lrand48(), 0, 1, 0); dotest(argv[1], lrand48(), 0, 0, 1); dotest(argv[1], lrand48(), 0, 1, 1); dotest(argv[1], lrand48(), 1, 0, 0); dotest(argv[1], lrand48(), 1, 1, 0); dotest(argv[1], lrand48(), 1, 0, 1); dotest(argv[1], lrand48(), 1, 1, 1); } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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