Messages in this thread | | | Date | Thu, 23 Feb 2006 20:13:38 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: IA64 non-contiguous memory space bugs |
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On Thu, 23 Feb 2006, 'David Gibson' wrote: > > Consider a HPAGE_SIZE hugepage VMA starting at 4GB, and a normal page > VMA starting at (4GB-PAGE_SIZE). This situation is possible on > powerpc, and is_hugepage_only_range(4GB-PAGE_SIZE, HPAGE_SIZE) will > (and must) return true. Therefore the free_pgtables() logic will call > hugetlb_free_pgd_range() across the normal page VMA.
Thanks for your patience, I eventually got it. Although (amused to observe my own incomprehension) I couldn't actually understand your explanation at all, realized it myself overnight, read again what you'd written, and then found that you had explained it very well.
Yes, I was wrong to use HPAGE_SIZE in that way in free_pgtables, and it ought to go to the trouble of testing the real end-addr (if we keep using is_hugepage_only_range there at all). Though it's nothing urgent while your hugetlb_free_pgd_range happens to be the same as your free_pgd_range, right? Is that changing soon?
May I plead the extenuating circumstance, that the powerpc is_hugepage_only_range means something quite different from the ia64? The ia64 one means "within a hugepage-only range" but the powerpc one means "overlaps a hugepage-only range"; I don't know which came first, and is_hugepage_only_range isn't very descriptive of either (though matches the ia64 case much better).
(That is, I think from the "touch" naming, and from your description, that the powerpc one means "overlaps". After a few minutes, I gave up trying to decipher exactly what LOW_ESID_MASK and HTLB_AREA_MASK end up doing, and take your superior knowledge on trust.)
While is_hugepage_only_range means different things to different architectures, I guess it'd best be avoided in common code. That use in get_unmapped_area: powerpc gets it right, but ia64 gets it wrong? But I didn't notice a change to that line (or the ia64 implementaton thereof) in your original patch.
> I can see two ways of fixing this. The quick, hacky fix is to use > is_vm_hugetlb_page(), and work around the problems by having > hugetlb_free_pgd_range() be identical to free_pgd_range() in most > cases.
I don't see that as hacky. I did point out that is_vm_hugetlb_page will miss out on some coalescence, but that can't be a big deal for what are already huge areas (the optimization was intended for many tiny adjacent areas).
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