Hi Cyril, On Mon, Nov 12, 2012 at 12:25 PM, <chrubis@xxxxxxx> wrote: > Hi! > This adds note about Linux behavior with partial page at the end of the > object. The problem here is that pages that contains only part of a > file (because the file size is not multiple of PAGE_SIZE) stays in page > cache even after the mapping is unmapped and the file closed. So if some > proces dirties such page other mappings will see the the changes rather > than zeroes. > > I've also attached reproducer which is a stripped down version of LTP > test. The child creates file of the size of PAGE_SIZE/2, maps it, > changes content after the PAGE_SIZE/2. The parent waits for child to > exit, maps the same file and checks the content after the PAGE_SIZE/2. > Uncommenting the msync() makes the test succeed. Thanks. And thanks also for the test program. I've applied this patch. Cheers, Michael > From 24dda667d02a128274133a60f8985bb061fe056c Mon Sep 17 00:00:00 2001 > From: Cyril Hrubis <chrubis@xxxxxxx> > Date: Wed, 7 Nov 2012 12:20:19 +0100 > Subject: [PATCH 2/2] mmap.2: Add note about partial page in BUGS > > Signed-off-by: Cyril Hrubis <chrubis@xxxxxxx> > --- > man2/mmap.2 | 9 +++++++++ > 1 files changed, 9 insertions(+), 0 deletions(-) > > diff --git a/man2/mmap.2 b/man2/mmap.2 > index 29fcc3d..5d2f7ee 100644 > --- a/man2/mmap.2 > +++ b/man2/mmap.2 > @@ -573,6 +573,15 @@ Since kernel 2.6.12, > fails with the error > .B EINVAL > for this case. > + > +POSIX specifies that system shall always zero fill any partial page at the end > +of the object and that system will newer write any modification of object > +beyond its end. On Linux when you write data to such partial page after the end > +of the object the data stays in page cache even after you closed and unmaped > +the file and although the data are newer written to the file itself subsequent > +mappings may see the modified content. In some cases this could be fixed by > +calling msync before the unmap takes place, this however doesn't work on tmpfs > +(for example when using shm ipc interface). > .SH EXAMPLE > .\" FIXME . Add an example here that uses an anonymous shared region for > .\" IPC between parent and child. > -- > 1.7.8.6 > > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface"; http://man7.org/tlpi/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
