Messages in this thread | | | Subject | Re: [GIT PULL] futex fixlet | From | Peter Zijlstra <> | Date | Fri, 30 Dec 2011 18:07:58 +0100 |
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On Thu, 2011-12-29 at 17:26 -0800, Linus Torvalds wrote: > Why do we even bother to check "page->mapping" at all? What's the > *use* of that loop? My gut feeling is that *that* is the fundamental > problem, and we should just get rid of it, rather than add all these > totally random work-arounds for the problem. > > Peter Z? That "if (!page->mapping) goto again" actually goes back to > 38d47c1b7075, in 2008. Why does it exist in the first place? There's > no comment nor explanation in the changelog. > > Why don't we just unconditionally return -EFAULT? What is the retry > actually supposed to *do*? If somebody races with a mmap/munmap, why > the hell would we care? What is it doing?
Vague memory seems to suggest it was to do with an unmap race, now the only such race we care about is swapping, if someone has a futex and does munmap+mmap under us we really don't care and you get to keep whatever result that yields.
That said, the ->mapping test is wrong because ->mapping is not actually cleared when the page is unmapped.
Also, I suspect the is_page_cache_freeable() test in pageout() avoids the worst of it. It keeps the page around if we have a reference to it, so a minor fault will then quickly re-instate the same page without loss of data.
So I _think_ you're completely right and we can simply kill the whole thing, but I've been trying very hard to forget everything kernel related for a week, and I really shouldn't kick-start my brain until somewhere next week.
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