From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hugh Dickins Date: Thu, 23 Feb 2006 20:13:38 +0000 Subject: Re: IA64 non-contiguous memory space bugs Message-Id: List-Id: References: <200602220132.k1M1Vxg09552@unix-os.sc.intel.com> <200602220151.k1M1pqg09761@unix-os.sc.intel.com> <20060222022558.GE23574@localhost.localdomain> <20060222234949.GB25108@localhost.localdomain> In-Reply-To: <20060222234949.GB25108@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: 'David Gibson' Cc: "Chen, Kenneth W" , linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org 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). Hugh