why does page_cache_pipe_buf_confirm() need to check page->mapping?

[Miklos Szeredi <miklos@szeredi.hu>]

Hi Jens,

I haven't done a great deal of research into this, but checking
page->mapping in page_cache_pipe_buf_confirm() might be bogus.

If the page is truncated *after* being spliced into the pipe, why on
earth does the buffer become invalid?

This looks to be a problem for filesystems that invalidate pages
(because the the data is possibly stale) and the pipe read returns
-ENODATA even though the data is there, it's just possibly different
from what was spliced into the pipe.  But I don't think that's a
reason for throwing away that buffer and definitely not a reason to
return an error.

Thanks,
Miklos

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