Messages in this thread | | | Date | Wed, 13 Dec 2006 03:02:38 -0800 | From | Suleiman Souhlal <> | Subject | Re: [patch 03/13] io-accounting: write accounting |
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Suleiman Souhlal wrote: > 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. > > > Ah yes indeed. I'm unable to keep up with all the new developments. :-( > > However, if my understanding of this code is correct, it seems that the > page fault is only done for shared writable VMAs, so you still can't > rely on set_page_dirty() always being called by the process that > dirtied the page in the first place. > > Am I wrong?
Yes I am. The only I/O non-shared VMAs might cause is from swapping, and I'm not sure if the io accounting patches actually care about that. My confusion came from the term "shared": A VMA is considered shared whenever MAP_SHARED is specified, even if it only has only one single "user".
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