From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by wa-out-1112.google.com with SMTP id m33so800588wag for ; Fri, 09 Nov 2007 11:15:32 -0800 (PST) Message-ID: <6934efce0711091115i3f859a00id0b869742029b661@mail.gmail.com> Date: Fri, 9 Nov 2007 11:15:32 -0800 From: "Jared Hulbert" Subject: [RFC] Changing VM_PFNMAP assumptions and rules MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline Sender: owner-linux-mm@kvack.org Return-Path: To: Linux Memory Management List List-ID: Per conversations regarding XIP from the vm/fs mini-summit a couple months back I've got a patch to air out. The basic problem is that the assumptions about PFN mappings stemming from the rules of remap_pfn_range() aren't always valid. For example: what stops one from using vm_insert_pfn() to map PFN's into a vma in an arbitrary order? Nothing. Yet those PFN's cause problems in two ways. First, vm_normal_page() won't return NULL. My answer to this is to simply check if pfn_valid() if it isn't then we've got a proper PFN that can only be a PFN. If you do have a valid PFN then you are (A) a 'cow'ed' PFN that is now a real page or (B) you are a real page pretending to be a PFN only. The thing that makes me nervous is that my hack doesn't let that page pretend to be a PFN. I can't figure out why a page would need/want to pretend to be a PFN so I don't see anything wrong with this, but maybe somebody does. Second, there are a few random BUG_ON() that don't seem to serve any purpose other than to punish the PFN's that don't abide by remap_pfn_range() rules. I just get rid of them. The problem is I don't really understand why they are there in the first place so for all I know I'm horribly breaking spufs or something. Okay so I haven't tried this out on 2.6.24-rc1 yet, but the same basic idea worked on 2.6.23 and older. I just wanted to get feedback on this approach. I don't know the vm all that well so I want to make sure I'm not doing something really stupid that breaks a bunch of code paths I don't use. diff --git a/mm/memory.c b/mm/memory.c index 9791e47..fb962d0 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -366,29 +366,19 @@ static inline int is_cow_mapping(unsigned int flags) * NOTE! Some mappings do not have "struct pages". A raw PFN mapping * will have each page table entry just pointing to a raw page frame * number, and as far as the VM layer is concerned, those do not have - * pages associated with them - even if the PFN might point to memory - * that otherwise is perfectly fine and has a "struct page". + * pages associated with them. * - * The way we recognize those mappings is through the rules set up - * by "remap_pfn_range()": the vma will have the VM_PFNMAP bit set, - * and the vm_pgoff will point to the first PFN mapped: thus every - * page that is a raw mapping will always honor the rule - * - * pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT) - * - * and if that isn't true, the page has been COW'ed (in which case it - * _does_ have a "struct page" associated with it even if it is in a - * VM_PFNMAP range). + * The old "remap_pfn_range()" rules don't work for all applications. + * Each "page" in a PFN mapping either has a page struct backing it + * or it doesn't. If it does then treat it like the page it is, if + * if it doesn't then it is not a normal page so just return NULL. */ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr, pte_t pte) { unsigned long pfn = pte_pfn(pte); if (unlikely(vma->vm_flags & VM_PFNMAP)) { - unsigned long off = (addr - vma->vm_start) >> PAGE_SHIFT; - if (pfn == vma->vm_pgoff + off) - return NULL; - if (!is_cow_mapping(vma->vm_flags)) + if (!pfn_valid(pfn)) return NULL; } @@ -1212,7 +1202,6 @@ int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr, spinlock_t *ptl; BUG_ON(!(vma->vm_flags & VM_PFNMAP)); - BUG_ON(is_cow_mapping(vma->vm_flags)); retval = -ENOMEM; pte = get_locked_pte(mm, addr, &ptl); @@ -2216,8 +2205,6 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma, vmf.flags = flags; vmf.page = NULL; - BUG_ON(vma->vm_flags & VM_PFNMAP); - if (likely(vma->vm_ops->fault)) { ret = vma->vm_ops->fault(vma, &vmf); if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org