[parisc-linux] Re: Problems with kernel mmap (failing tst-mmap-eofsync in glibc on parisc)

[James Bottomley]

On Fri, 2003-08-22 at 14:19, David S. Miller wrote:
> Sparc64's alias'able caches are 1) write-through and 2) quite small.
> 
> I think I begin to see the issue clearly now.
> 
> But you cannot do the VM_SHARED change without an audit first.
> Lots of code thinks that VM_SHARED means someone maybe wrote
> to the page through a mmap().  For example look at how filemap
> sync interprets this flag bit.

Yes, the issue seems to be that the flush_dcache_page() was implemented
with the thought that the caches of the shared mappings may contain
modified data that needs to be flushed to the aliased page.

The opposite property: that the caches of the aliased page need to be
invalidated because someone else changed data in the aliased page seems
to work as a byproduct of the above implementation.

But some of the checks for !list_empty(&mapping->i_shared) are going to
prevent the necessary invalidations on read only shared mappings...which
was the initial problem.

The only issue I can see with not dropping VM_SHARED for read only
shared mappings is that we do spurious (but harmless)
flushe_dcache_page() on reads.

There also appears to be a lurking prob lem in do_mremap, where it keys
off the VM_SHARED flag to set the MAP_SHARED flag for
get_unmapped_area.  That's going to cause us a problem on parisc because
SHARED pages need to obey slightly stricter alignment constraints

All in all, I think not dropping VM_SHARED on read only shared mappings
is the right thing to do.

Do you need a more detailed audit?

James