On Tue, 2006-12-19 at 00:04 -0800, Linus Torvalds wrote:
> Nobody has actually ever explained why "test_clear_page_dirty()" is good
> at all.
>
> - Why is it ever used instead of "clear_page_dirty_for_io()"?
>
> - What is the difference?
>
> - Why would you EVER want to clear bits just in the "struct page *" or
> just in the PTE's?
>
> - Why is it EVER correct to clear dirty bits except JUST BEFORE THE IO?
>
> In other words, I have a theory:
>
> "A lot of this is actually historical cruft. Some of it may even be code
> that was never supposed to work, but because we maintained _other_ dirty
> bits in the PTE's, and never touched them before, we never even realized
> that the code that played with PG_dirty was totally insane"
>
> Now, that's just a theory. And yeah, it may be stated a bit provocatively.
> It may not be entirely correct. I'm just saying.. maybe it is?
On Sun, 2006-12-17 at 15:40 -0800, Andrew Morton wrote:
> try_to_free_buffers() clears the page's dirty state if it successfully removed
> the page's buffers.
>
> Background for this:
>
> - a process does a one-byte-write to a file on a 64k pagesize, 4k
> blocksize ext3 filesystem. The page is now PageDirty, !PgeUptodate and
> has one dirty buffer and 15 not uptodate buffers.
>
> - kjournald writes the dirty buffer. The page is now PageDirty,
> !PageUptodate and has a mix of clean and not uptodate buffers.
>
> - try_to_free_buffers() removes the page's buffers. It MUST now clear
> PageDirty. If we were to leave the page dirty then we'd have a dirty, not
> uptodate page with no buffer_heads.
>
> We're screwed: we cannot write the page because we don't know which
> sections of it contain garbage. We cannot read the page because we don't
> know which sections of it contain modified data. We cannot free the page
> because it is dirty.
However!! this is not true for mapped pages because mapped pages must
have the whole (16k in akpm's example) page loaded. Hence I suspect that
what Andrei did by accident - remove the if (mapping) case in
test_clean_dirty_pages() - is actually totally correct.
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