On Fri, Feb 20, 2015 at 08:16:32PM -0800, Hugh Dickins wrote: > Andrea wrote a very interesting comment on THP in mm/memory.c, > just before the end of __handle_mm_fault(): > > * A regular pmd is established and it can't morph into a huge pmd > * from under us anymore at this point because we hold the mmap_sem > * read mode and khugepaged takes it in write mode. So now it's > * safe to run pte_offset_map(). > > This comment hints at several difficulties, which anon THP solved > for itself with mmap_sem and anon_vma lock, but which huge tmpfs > may need to solve differently. > > The reference to pte_offset_map() above: I believe that's a hint > that on a 32-bit machine, the pagetables might need to come from > kernel-mapped memory, but a huge pmd pointing to user memory beyond > that limit could be racily substituted, causing undefined behavior > in the architecture-dependent pte_offset_map(). > > That itself is not a problem on x86_64, but there's plenty more: > how about those places which use pte_offset_map_lock() - if that > spinlock is in the struct page of a pagetable, which has been > deposited and might be withdrawn and freed at any moment (being > on a list unattached to the allocating pmd in the case of x86), > taking the spinlock might corrupt someone else's struct page. > > Because THP has departed from the earlier rules (when pagetable > was only freed under exclusive mmap_sem, or at exit_mmap, after > removing all affected vmas from the rmap list): zap_huge_pmd() > does pte_free() even when serving MADV_DONTNEED under down_read > of mmap_sem. > > And what of the "entry = *pte" at the start of handle_pte_fault(), > getting the entry used in pte_same(,orig_pte) tests to validate all > fault handling? If that entry can itself be junk picked out of some > freed and reused pagetable, it's hard to estimate the consequences. > > We need to consider the safety of concurrent faults, and the > safety of rmap lookups, and the safety of miscellaneous operations > such as smaps_pte_range() for reading /proc/<pid>/smaps. > > I set out to make safe the places which descend pgd,pud,pmd,pte, > using more careful access techniques like mm_find_pmd(); but with > pte_offset_map() being architecture-defined, it's too big a job to > tighten it up all over. > > Instead, approach from the opposite direction: just do not expose > a pagetable in an empty *pmd, until vm_ops->fault has had a chance > to ask for a huge pmd there. This is a much easier change to make, > and we are lucky that all the driver faults appear to be using > interfaces (like vm_insert_page() and remap_pfn_range()) which > automatically do the pte_alloc() if it was not already done. > > But we must not get stuck refaulting: need FAULT_FLAG_MAY_HUGE for > __do_fault() to tell shmem_fault() to try for huge only when *pmd is > empty (could instead add pmd to vmf and let shmem work that out for > itself, but probably better to hide pmd from vm_ops->faults). > > Without a pagetable to hold the pte_none() entry found in a newly > allocated pagetable, handle_pte_fault() would like to provide a static > none entry for later orig_pte checks. But architectures have never had > to provide that definition before; and although almost all use zeroes > for an empty pagetable, a few do not - nios2, s390, um, xtensa. > > Never mind, forget about pte_same(,orig_pte), the three __do_fault() > callers can follow do_anonymous_page()'s example, and just use a > pte_none() check instead - supplemented by a pte_file pte_to_pgoff > check until the day VM_NONLINEAR is removed. > > do_fault_around() presents one last problem: it wants pagetable to > have been allocated, but was being called by do_read_fault() before > __do_fault(). But I see no disadvantage to moving it after, > allowing huge pmd to be chosent first. One disadvantage is addtional radix-tree lookup for page cache hot case. IIRC, the difference was small, but measurable back when I implemented faultaround. Have you considered pushing page table allocation even futher -- into do_set_pte()? -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>