Messages in this thread | | | Date | Wed, 23 Mar 2005 00:51:02 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: [PATCH 1/5] freepgt: free_pgtables use vma list |
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On Tue, 22 Mar 2005, David S. Miller wrote: > On Tue, 22 Mar 2005 21:51:39 +0000 (GMT) > Hugh Dickins <hugh@veritas.com> wrote: > > > I still can't see what's wrong with the code that's already > > there. My brain is seizing up, I'm taking a break. > > Ok, meanwhile I'll do a brain dump of what I think this > code should be doing. > > Let's take an example free_pgd_range() call. Say the > address parameters are: > > addr 0x10000 > end 0xa4000 > floor 0x00000 > ceiling 0xb2000
This actual example helped to focus my mind a lot, thank you.
> (This example comes from my exit_mmap() VMA dump earlier > in this thread. If you disable the VMA skipping optimization > the first call to free_pgd_range() has these parameters.) > > What ought this free_pgd_range() call do? This range of > addresses, from floor to ceiling, is smaller than a PMD_SIZE > (which on sparc64 is 1 << 23). Therefore it should clear > no PGD or PUD entries.
Yup, it ought to decide at the beginning of free_pgd_range that it simply has no work to do.
> Yet, it does clear them, specifically: > > free_pgd_range(): > 1) mask addr (0x10000) to PMD_MASK, addr is now 0 > 2) addr < floor (0x00000) test does not pass > 3) mask ceiling (0xb2000) to PMD_MASK, ceiling is now 0 too > 4) end - 1 > ceiling - 1 test does not pass > 5) addr > end - 1 test does not pass either
And now we've gone wrong, yes.
> The source of the problems seems to be how ceiling began > at the top of the call chain as 0xb2000, but when we > masked it with PMD_MASK that set it to zero, which means > "top of address space" in these functions. That's not > what we want. > > I added a quick hack to the simulator I posted, where > we mask ceiling in free_pgd_range(), I do it like this: > > if (ceiling) { > ceiling &= PMD_MASK; > if (!ceiling) > return; > }
At first that just looked like a hack to me. But on reflection, no, you're doing exactly what I had to do with addr above: in the case where we arrive at 0 from non-0 value, have to get out quick to avoid confusion with the "other" 0. These wrap issues are hard.
And in other mail I see you found more such checks were needed. I believe you've got it, thank you so much!
Though frankly, by now, I'm sure of nothing: will review in the morning.
> and things seem to behave. I'll try to analyze things > further and test this out on a real kernel, but all of > these adjustments at the top of free_pgd_range() really > start to look like pure spaghetti. :-)
Well, it's trying to decide in reasonably few steps that it's not worth wasting time going down to the deeper levels. Lots of "return"s as it eliminates cases, yes.
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