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SubjectRe: [parisc-linux] Re: Problems with kernel mmap (failing tst-mmap-eofsync in glibc on parisc)
On Sat, 2003-08-23 at 16:43, David S. Miller wrote:
> On 22 Aug 2003 20:09:30 -0500
> James Bottomley <> wrote:
> > What we were hoping is that we could rely on this little property of
> > mmap:
> >
> > Create a private copy-on-write mapping. Stores
> > to the region do not affect the original file.
> > It is unspecified whether changes made to the
> > file after the mmap call are visible in the
> > mapped region.
> >
> > To avoid having to flush the non-shared mappings (basically on parisc if
> > you write to a file backing a MAP_PRIVATE mapping then we don't
> > guarantee you see the update).
> >
> > I suppose if we had a way of telling if any of the i_mmap list members
> > were really MAP_SHARED semantics mappings, then we could alter our
> > flush_dcache_page() implementation to work.
> I thought about this very deeply last night and this morning.
> And what you're trying to optimize won't work. Here is why.
> If the first access to a MAP_PRIVATE mapping of a page is a read,
> we'll use the page-cache page. This means that, with your
> optimization, during this time if another cpu write()`s into the
> page we'll lose the data update.
> Sorry :(

Could you elaborate some more? I agree that the MAP_PRIVATE mapping may
not see cpu1's write because of cache incoherencies (but that's what I
believe is covered by the `unspecified' bit of the MAP_PRIVATE
definition above). MAP_PRIVATE is COW (i.e. the page is marked read
only while it's shared), so there can be no data to flush in the cache
for the page of the MAP_PRIVATE process...The only scenario I see where
we can get cache based data destruction is if two cache aliases both
contain dirty caches for the page (which can only happen for MAP_SHARED
RW, which we already have the correct semantics for...they're racing to
do a msync and the last one in wins).


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