Messages in this thread | | | Subject | Re: [patch 03/13] io-accounting: write accounting | From | Peter Zijlstra <> | Date | Wed, 13 Dec 2006 11:04:54 +0100 |
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On Wed, 2006-12-13 at 00:59 -0800, Andrew Morton wrote: > On Wed, 13 Dec 2006 00:45:50 -0800 > Suleiman Souhlal <ssouhlal@FreeBSD.org> wrote: > > > akpm@osdl.org wrote: > > > From: Andrew Morton <akpm@osdl.org> > > > > > > Accounting writes is fairly simple: whenever a process flips a > page from clean > > > to dirty, we accuse it of having caused a write to underlying > storage of > > > PAGE_CACHE_SIZE bytes. > > > > On architectures where dirtying a page doesn't cause a page fault > (like i386), couldn't you end up billing the wrong process (in fact, I > think that even on other archituctures set_page_dirty() doesn't get > called immediately in the page fault handler)? > > Yes, that would be a problem in 2.6.18 and earlier. > > In 2.6.19 and later, we do take a fault when transitioning a page from > pte-clean to pte-dirty. That was done to get the dirty-page > accounting > right - to avoid the > all-of-memory-is-dirty-but-the-kernel-doesn't-know-it > problem. > > > > AFAICS, set_page_dirty() is mostly called when trying to unmap a > page when trying to shrink LRU lists, and there is no guarantee that > this happens under the process that dirtied it (in fact, the > set_page_dirty() is often done by kswapd).
we're talking:
shrink_page_list() TestSetPageLock() try_to_unmap() try_to_unmap_file() try_to_unmap_one() set_page_dirty()
Which, because its run under the page lock, should always be false, right? perhaps a WARN_ON might do.
> hm, that code is still there in zap_pte_range(). If all is well, that > set_page_dirty() call should never return true.
exit_mmap(), do_unmap() unmap_vmas() unmap_page_range() zap_{pud,pmd,pte}_range()
vmtruncate() unmap_mapping_range() unmap_mapping_range_*() unmap_mapping_range_vma(), madvise_dontneed() zap_page_range() unmap_vmas() unmap_page_range() zap_{pud,pmd,pte}_range()
Which is all ran without the page lock, so might be open for a race; however, I don't see that hurting because the whole page is un-accounted pretty soon afterwards.
> Peter did, you ever test for that?
Nope
> (Well, it might return true in rare races, because zap_pte_range() > doesn't lock the pages)
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