Hi,
I just took a glance, maybe due to
commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface")
I simply reverted it just now and it seems fine... but I haven't digged into this commit.
Maybe you could Cc NeilBrown <neilb@xxxxxxxx> for some more advice and
I have no idea whether it's an expected behavior or not...
Thanks,
Gao Xiang
On 2019/8/1 14:16, Sergei Turchanov wrote:
> Hello!
>
> (I sent this e-mail two weeks ago with no feedback. Does anyone care? Wrong mailing list? Anything....?)
>
> Seeking (to an offset within file size) in /proc/meminfo is broken in 4.19.59. It does seek to a desired position, but reading from that position returns the remainder of file and then a whole copy of file. This doesn't happen with /proc/vmstat or /proc/self/maps for example.
>
> Seeking did work correctly in kernel 4.14.47. So it seems something broke in the way.
>
> Background: this kind of access pattern (seeking to /proc/meminfo) is used by libvirt-lxc fuse driver for virtualized view of /proc/meminfo. So that /proc/meminfo is broken in guests when running kernel 4.19.x.
>
> $ ./test /proc/meminfo 0 # Works as expected
>
> MemTotal: 394907728 kB
> MemFree: 173738328 kB
> ...
> DirectMap2M: 13062144 kB
> DirectMap1G: 390070272 kB
>
> -----------------------------------------------------------------------
>
> $ ./test 1024 # returns a copy of file after the remainder
>
> Will seek to 1024
>
>
> Data read at offset 1024
> gePages: 0 kB
> ShmemHugePages: 0 kB
> ShmemPmdMapped: 0 kB
> HugePages_Total: 0
> HugePages_Free: 0
> HugePages_Rsvd: 0
> HugePages_Surp: 0
> Hugepagesize: 2048 kB
> Hugetlb: 0 kB
> DirectMap4k: 245204 kB
> DirectMap2M: 13062144 kB
> DirectMap1G: 390070272 kB
> MemTotal: 394907728 kB
> MemFree: 173738328 kB
> MemAvailable: 379989680 kB
> Buffers: 355812 kB
> Cached: 207216224 kB
> ...
> DirectMap2M: 13062144 kB
> DirectMap1G: 390070272 kB
>
> As you see, after "DirectMap1G:" line, a whole copy of /proc/meminfo returned by "read".
>
> Test program:
>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
>
> #define SIZE 1024
> char buf[SIZE + 1];
>
> int main(int argc, char *argv[]) {
> int fd;
> ssize_t rd;
> off_t ofs = 0;
>
> if (argc < 2) {
> printf("Usage: test <file> [<offset>]\n");
> exit(1);
> }
>
> if (-1 == (fd = open(argv[1], O_RDONLY))) {
> perror("open failed");
> exit(1);
> }
>
> if (argc > 2) {
> ofs = atol(argv[2]);
> }
> printf("Will seek to %ld\n", ofs);
>
> if (-1 == (lseek(fd, ofs, SEEK_SET))) {
> perror("lseek failed");
> exit(1);
> }
>
> for (;; ofs += rd) {
> printf("\n\nData read at offset %ld\n", ofs);
> if (-1 == (rd = read(fd, buf, SIZE))) {
> perror("read failed");
> exit(1);
> }
> buf[rd] = '\0';
> printf(buf);
> if (rd < SIZE) {
> break;
> }
> }
>
> return 0;
> }
>
>
>
