Hi!
I'm currently revisiting mmap related tests in LTP (Linux Test Project)
and I've came to the tests testing that writes to the partially
mapped page (at the end of mapping) are carried out correctly.
These tests fails because even after the object is unmapped and the
file-descriptor closed the pages still stays in the page cache so if
(possibly another process) opens and maps the file again the whole
content of the partial page is preserved.
Strictly speaking this is not a bug at least when sticking to regular
files as POSIX which says that the change is not written out. In this
case the file content is correct and forcing the data to be written out
by msync() makes the test pass. The SHM mappings seems to preserve the
content even after calling msync() which is, in my opinion, POSIX
violation although a minor one.
Looking at the test results I have, the file based mmap test worked fine
on 2.6.5 (or perhaps the page cache was working/setup differently and
the test succeeded by accidend).
Attached is a stripped down LTP test for the problem, uncommenting the
msync() makes the test succeed.
I would like to hear your opinions on this problems.
--
Cyril Hrubis
[email protected]
On Wed, Oct 31, 2012 at 2:24 AM, <[email protected]> wrote:
> Hi!
> I'm currently revisiting mmap related tests in LTP (Linux Test Project)
> and I've came to the tests testing that writes to the partially
> mapped page (at the end of mapping) are carried out correctly.
>
> These tests fails because even after the object is unmapped and the
> file-descriptor closed the pages still stays in the page cache so if
> (possibly another process) opens and maps the file again the whole
> content of the partial page is preserved.
>
> Strictly speaking this is not a bug at least when sticking to regular
> files as POSIX which says that the change is not written out. In this
> case the file content is correct and forcing the data to be written out
> by msync() makes the test pass. The SHM mappings seems to preserve the
> content even after calling msync() which is, in my opinion, POSIX
> violation although a minor one.
>
fsync implemented in SHM is noop_fsync.
May be we should extend it if needed.
> Looking at the test results I have, the file based mmap test worked fine
> on 2.6.5 (or perhaps the page cache was working/setup differently and
> the test succeeded by accidend).
>
> Attached is a stripped down LTP test for the problem, uncommenting the
> msync() makes the test succeed.
>
> I would like to hear your opinions on this problems.
>
> --
> Cyril Hrubis
> [email protected]
--
Regards,
--Bob
Hi!
> > Strictly speaking this is not a bug at least when sticking to regular
> > files as POSIX which says that the change is not written out. In this
> > case the file content is correct and forcing the data to be written out
> > by msync() makes the test pass. The SHM mappings seems to preserve the
> > content even after calling msync() which is, in my opinion, POSIX
> > violation although a minor one.
> >
>
> fsync implemented in SHM is noop_fsync.
> May be we should extend it if needed.
I'm entirely sure that would fix the interface correclty. The posix
says:
mmap:
...
The system shall always zero-fill any partial page at the end of an
object. Further, the system shall never write out any modified portions
of the last page of an object which are beyond its end.
...
msync:
...
The effect of msync() on a shared memory object or a typed memory object
is unspecified.
...
Hmm, that is a little confusing and it looks like it depends on
interpretation what 'write out' for SHM object means. And I guess that
leaving the SHM part as it is is reasonable. Maybe worth of a note in
manual page.
On the other hand there seems to be several bugs in mmap() on regular
files. For example mapping half of the page from a file doesn't fill the
rest of the page with zeroes. And it looks like when half of page is
mapped, the second half modified, then unmapped and then the whole page
is mapped the content doesn't seem to be right, although this seems to
change randomly. The reproducer for the first case attached.
--
Cyril Hrubis
[email protected]