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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
