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SubjectRe: [parisc-linux] Re: Problems with kernel mmap (failing tst-mmap-eofsync in glibc on parisc)
On 22 Aug 2003 20:09:30 -0500
James Bottomley <James.Bottomley@SteelEye.com> wrote:

> What we were hoping is that we could rely on this little property of
> mmap:
>
> MAP_PRIVATE
> 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 :(
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