Messages in this thread | | | Date | Thu, 8 May 2008 10:16:57 -0700 | From | Nishanth Aravamudan <> | Subject | Re: [PATCH] x86: fix PAE pmd_bad bootup warning |
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On 08.05.2008 [18:51:11 +0200], Hans Rosenfeld wrote: > On Thu, May 08, 2008 at 09:33:52AM -0700, Nishanth Aravamudan wrote: > > So this seems to lend credence to Dave's hypothesis. Without, as you > > were trying before, teaching pagemap all about hugepages, what are our > > options? > > > > Can we just skip over the current iteration of the PMD loop (would we > > need something similar for the PTE loop for power?) if pmd_huge(pmd)? > > Allowing huge pages in the page walker would affect both walk_pmd_range > and walk_pud_range. Then either the users of the page walker need to > know how to handle huge pages themselves (in the pmd_entry and pud_entry > callback functions), or the page walker treats huge pages as any other > pages (calling the pte_entry callback function).
Right, I agree *if* we allow huge pages in the walker. But AIUI, things are broken now with hugepages in the process' address space. This is a bug upstream and leads to hugepages leaking out of the kernel when /proc/pid/pagemap is read. Why not, instead (as a short-term fix), skip hugepage mappings altogether in the page-walker code?
Hrm, upon further investigation, this seems to be a pretty clear limitation of walk_page_range(). One that is avoided in the other two callers, i.e.
static int show_smap(struct seq_file *m, void *v) { ... if (vma->vm_mm && !is_vm_hugetlb_page(vma)) walk_page_range(vma->vm_mm, vma->vm_start, vma->vm_end, &smaps_walk, &mss); ... }
static ssize_t clear_refs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos) { ... for (vma = mm->mmap; vma; vma = vma->vm_next) if (!is_vm_hugetlb_page(vma)) walk_page_range(mm, vma->vm_start, vma->vm_end, &clear_refs_walk, vma); ... }
No such protection exists for
static ssize_t pagemap_read(struct file *file, char __user *buf, size_t count, loff_t *ppos);
So, is there any way to either add a is_vm_hugetlb_page(vma) check into pagemap_read()? Or can we modify walk_page_range to take the a vma and skip the walking if is_vm_hugetlb_page(vma) is set [to avoid complications down the road until hugepage walking is fixed]. I guess the latter isn't possible for pagemap_read(), since we are just looking at arbitrary addresses in the process space?
Dunno, seems quite clear that the bug is in pagemap_read(), not any hugepage code, and that the simplest fix is to make pagemap_read() do what the other walker-callers do, and skip hugepage regions.
Thanks, Nish
-- Nishanth Aravamudan <nacc@us.ibm.com> IBM Linux Technology Center
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