On Mon, 5 Mar 2007 13:43:03 -0800 (PST)
Linus Torvalds <torvalds@linux-foundation.org> wrote:
The thing about nobh mode is that because we have no buffer_heads, we can't
track the uptodateness of sections of the page. Hence nobh pages are
basically always uptodate. The only place where we can tolerate partial
uptodateness is in between prepare_write and commit_write, where we omit
the initialisation of the section of the page which the caller is writing
to.
Of course, this won't perform very well with 64k pages..
nobh_prepare_write brings uptodate the sections of the page (0->from) and
(to->PAGE_CACHE_SIZE), and the nobh_prepare_write() caller brings the
(from->to) section uptodate. So the page is uptodate at
nobh_commit_write(). It has to be, because we don't know how to bring a
non-uptodate nobh page uptodate apart from writing something to every byte
in it.
It's not really appropriate that nobh_truncate_page() call
->prepare_write() at all. But it just happened that nobh_prepare_write()
does exactly what nobh_truncate_page() wants to do, so I just called
nobh_prepare_write() for code-sharing reasons.
Perhaps I should have called nobh_prepare_write() directly, or created some
common private function which both nobh_prepare_write() and
nobh_truncate_page() internally call.
Is OK, I think. nobh_prepare_write() brings the outside-from-and-to
sections of the page uptodate and memset in nobh_truncate_page() brings the
rest of the page uptodate.
We bring the to->PAGE_CACHE_SIZE section uptodate twice, which could be
optimised.
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