Date: Mon, 14 Jun 1999 00:07:29 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: "David E. Cross" <crossd@cs.rpi.edu> Cc: The Hermit Hacker <scrappy@hub.org>, freebsd-current@FreeBSD.ORG, freebsd-stable@FreeBSD.ORG, crossd@cs.rpi.edu Subject: Re: MMAP() in STABLE/CURRENT ... Message-ID: <199906140707.AAA06800@apollo.backplane.com> References: <199906140122.VAA02659@cs.rpi.edu>
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:Here's the code:
:
:#include <stdio.h>
:#include <stdlib.h>
:#include <unistd.h>
:#include <fcntl.h>
:#include <sys/types.h>
:#include <sys/mman.h>
:
:#define DBSIZE 733055625
:
:int main(void)
:{
: int fd;
: unsigned char *dbp;
:
: fd=open(argv[1], O_CREAT | O_RDWR | O_TRUNC, 0600);
: lseek(fd, DBSIZE -1, SEEK_SET);
: write(fd, &fd, 1);
: dbp=mmap(NULL, DBSIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
: close (fd);
: memset(dbp, 0, DBSIZE);
: munmap(dbp, DBSIZE);
: return 0;
:}
:}
Someone asked me at USENIX what would happen if the filesystem filled
up due to someone mmap'ing a file shared and then filled in its holes
by modifying the mmaping.
In your tests, are you intentionally filling up the filesystem? You
appear to be writing a rather large file - 733MB.
I'm just trying to narrow down where the bug is... the system isn't
supposed to panic or deadlock even if you do fill up the filesystem.
Your test program may also be hitting another known bug: If you dirty
the pages underlying a mapped file the system does not currently limit
the number of pages that can be dirtied. It is possible to eat up all
available memory and cause a deadlock to occur when the system is unable
to write the pages out to the file due to blocking on the access to the
file's indirect blocks. Actually, I think this is the more likely
scenario. This bug is already on my list.
-Matt
Matthew Dillon
<dillon@backplane.com>
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