From: Michal Simek <michal.simek@xxxxxxxxxxxxx> Date: Wed, 01 Dec 2010 18:10:12 +0100 > Roland McGrath wrote: >> This is a VM question more than a ptrace question. I can't give you >> any authoritative answers about the VM issues. >> Documentation/cachetlb.txt says: >> Any time the kernel writes to a page cache page, _OR_ >> the kernel is about to read from a page cache page and >> user space shared/writable mappings of this page potentially >> exist, this routine is called. >> In your case, the kernel is only reading (write=0 passed to >> access_process_vm and get_user_pages). In normal situations, >> the page in question will have only a private and read-only >> mapping in user space. So the call should not be required in >> these cases--if the code can tell that's so. >> Perhaps something like the following would be safe. >> But you really need some VM folks to tell you for sure. >> diff --git a/mm/memory.c b/mm/memory.c >> index 02e48aa..2864ee7 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -1484,7 +1484,8 @@ int __get_user_pages(struct task_struct *tsk, >> struct mm_struct *mm, >> pages[i] = page; >> flush_anon_page(vma, page, start); >> - flush_dcache_page(page); >> + if ((vm_flags & VM_WRITE) || (vma->vm_flags & VM_SHARED) >> + flush_dcache_page(page); >> } >> if (vmas) >> vmas[i] = vma; >> Thanks, >> Roland > > Andrew any comment? I don't have any comments on this specific patch but I will note that special care is needed _after_ the access to kick out aliases so that other future accesses to this page, which are oblivious to what ptrace did, don't see illegal D-cache aliases. Have a look at arch/sparc/kernel/ptrace_64.c:flush_ptrace_access(). Also, another issue is that much of the time ptrace() is just fetching very small chunks (perhaps, a stack frame, or some variable in the program image), so doing an entire page flush when we only copy a few bytes out of the page is overkill. Sparc64's flush_ptrace_access() tries to address this as well. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>