Re: [patch 02/14] tmpfs: fix regressions from wider use of ZERO_PAGE

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Thu, Apr 21, 2022 at 8:06 AM Borislav Petkov <bp@alien8.de> wrote:
>
> on AMD zen3
>
> original:       20.11 Gb/s
> rep_good:       34.662 Gb/s
> erms:           36.378 Gb/s
> fsrm:           36.398 Gb/s

Looks good.

Of course, the interesting cases are the "took a page fault in the middle" ones.

A very simple basic test is something like the attached.

It does no error checking or anything else, but doing a 'strace
./a.out' should give you something like

  ...
  openat(AT_FDCWD, "/dev/zero", O_RDONLY) = 3
  mmap(NULL, 196608, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0) = 0x7f10ddfd0000
  munmap(0x7f10ddfe0000, 65536)           = 0
  read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 65536) = 16
  exit_group(16)                          = ?

where that "read(..) = 16" is the important part. It correctly figured
out that it can only do 16 bytes (ok, 17, but we've always allowed the
user accessor functions to block).

With erms/fsrm, presumably you get that optimal "read(..) = 17".

I'm sure we have a test-case for this somewhere, but it was easier for
me to write a few lines of (bad) code than try to find it.

               Linus

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#include <stddef.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>

// Whatever
#define PAGE_SIZE 65536

int main(int argc, char **argv)
{
	int fd;
	void *map;

	fd = open("/dev/zero", O_RDONLY); 

	map = mmap(NULL, 3*PAGE_SIZE, PROT_READ | PROT_WRITE,
		MAP_PRIVATE |MAP_ANONYMOUS , -1, 0);
	munmap(map + PAGE_SIZE, PAGE_SIZE);

	return read(fd, map + PAGE_SIZE - 17, PAGE_SIZE);
}