On Tue, 13 Mar 2018, Laurent Dufour wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> When speculating faults (without holding mmap_sem) we need to validate
> that the vma against which we loaded pages is still valid when we're
> ready to install the new PTE.
>
> Therefore, replace the pte_offset_map_lock() calls that (re)take the
> PTL with pte_map_lock() which can fail in case we find the VMA changed
> since we started the fault.
>
Based on how its used, I would have suspected this to be named
pte_map_trylock().
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>
> [Port to 4.12 kernel]
> [Remove the comment about the fault_env structure which has been
> implemented as the vm_fault structure in the kernel]
> [move pte_map_lock()'s definition upper in the file]
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
> ---
> include/linux/mm.h | 1 +
> mm/memory.c | 56 ++++++++++++++++++++++++++++++++++++++----------------
> 2 files changed, 41 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4d02524a7998..2f3e98edc94a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -300,6 +300,7 @@ extern pgprot_t protection_map[16];
> #define FAULT_FLAG_USER 0x40 /* The fault originated in userspace */
> #define FAULT_FLAG_REMOTE 0x80 /* faulting for non current tsk/mm */
> #define FAULT_FLAG_INSTRUCTION 0x100 /* The fault was during an instruction fetch */
> +#define FAULT_FLAG_SPECULATIVE 0x200 /* Speculative fault, not holding mmap_sem */
>
> #define FAULT_FLAG_TRACE \
> { FAULT_FLAG_WRITE, "WRITE" }, \
I think FAULT_FLAG_SPECULATIVE should be introduced in the patch that
actually uses it.
> diff --git a/mm/memory.c b/mm/memory.c
> index e0ae4999c824..8ac241b9f370 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2288,6 +2288,13 @@ int apply_to_page_range(struct mm_struct *mm, unsigned long addr,
> }
> EXPORT_SYMBOL_GPL(apply_to_page_range);
>
> +static bool pte_map_lock(struct vm_fault *vmf)
inline?
> +{
> + vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> + vmf->address, &vmf->ptl);
> + return true;
> +}
> +
> /*
> * handle_pte_fault chooses page fault handler according to an entry which was
> * read non-atomically. Before making any commitment, on those architectures
> @@ -2477,6 +2484,7 @@ static int wp_page_copy(struct vm_fault *vmf)
> const unsigned long mmun_start = vmf->address & PAGE_MASK;
> const unsigned long mmun_end = mmun_start + PAGE_SIZE;
> struct mem_cgroup *memcg;
> + int ret = VM_FAULT_OOM;
>
> if (unlikely(anon_vma_prepare(vma)))
> goto oom;
> @@ -2504,7 +2512,11 @@ static int wp_page_copy(struct vm_fault *vmf)
> /*
> * Re-check the pte - we dropped the lock
> */
> - vmf->pte = pte_offset_map_lock(mm, vmf->pmd, vmf->address, &vmf->ptl);
> + if (!pte_map_lock(vmf)) {
> + mem_cgroup_cancel_charge(new_page, memcg, false);
> + ret = VM_FAULT_RETRY;
> + goto oom_free_new;
> + }
Ugh, but we aren't oom here, so maybe rename oom_free_new so that it makes
sense for return values other than VM_FAULT_OOM?
> if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> if (old_page) {
> if (!PageAnon(old_page)) {
> @@ -2596,7 +2608,7 @@ static int wp_page_copy(struct vm_fault *vmf)
> oom:
> if (old_page)
> put_page(old_page);
> - return VM_FAULT_OOM;
> + return ret;
> }
>
> /**
> @@ -2617,8 +2629,8 @@ static int wp_page_copy(struct vm_fault *vmf)
> int finish_mkwrite_fault(struct vm_fault *vmf)
> {
> WARN_ON_ONCE(!(vmf->vma->vm_flags & VM_SHARED));
> - vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, vmf->address,
> - &vmf->ptl);
> + if (!pte_map_lock(vmf))
> + return VM_FAULT_RETRY;
> /*
> * We might have raced with another page fault while we released the
> * pte_offset_map_lock.
> @@ -2736,8 +2748,11 @@ static int do_wp_page(struct vm_fault *vmf)
> get_page(vmf->page);
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> lock_page(vmf->page);
> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> - vmf->address, &vmf->ptl);
> + if (!pte_map_lock(vmf)) {
> + unlock_page(vmf->page);
> + put_page(vmf->page);
> + return VM_FAULT_RETRY;
> + }
> if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> unlock_page(vmf->page);
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -2947,8 +2962,10 @@ int do_swap_page(struct vm_fault *vmf)
> * Back out if somebody else faulted in this pte
> * while we released the pte lock.
> */
Comment needs updating, pte_same() isn't the only reason to bail out here.
> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> - vmf->address, &vmf->ptl);
> + if (!pte_map_lock(vmf)) {
> + delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
> + return VM_FAULT_RETRY;
> + }
> if (likely(pte_same(*vmf->pte, vmf->orig_pte)))
> ret = VM_FAULT_OOM;
> delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
Not crucial, but it would be nice if this could do goto out instead,
otherwise this is the first mid function return.
> @@ -3003,8 +3020,11 @@ int do_swap_page(struct vm_fault *vmf)
> /*
> * Back out if somebody else already faulted in this pte.
> */
Same as above.
> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> - &vmf->ptl);
> + if (!pte_map_lock(vmf)) {
> + ret = VM_FAULT_RETRY;
> + mem_cgroup_cancel_charge(page, memcg, false);
> + goto out_page;
> + }
> if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte)))
> goto out_nomap;
>
mem_cgroup_try_charge() is done before grabbing pte_offset_map_lock(), why
does the out_nomap exit path do mem_cgroup_cancel_charge();
pte_unmap_unlock()? If the pte lock can be droppde first, there's no need
to embed the mem_cgroup_cancel_charge() here.
> @@ -3133,8 +3153,8 @@ static int do_anonymous_page(struct vm_fault *vmf)
> !mm_forbids_zeropage(vma->vm_mm)) {
> entry = pte_mkspecial(pfn_pte(my_zero_pfn(vmf->address),
> vma->vm_page_prot));
> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> - vmf->address, &vmf->ptl);
> + if (!pte_map_lock(vmf))
> + return VM_FAULT_RETRY;
> if (!pte_none(*vmf->pte))
> goto unlock;
> ret = check_stable_address_space(vma->vm_mm);
> @@ -3169,8 +3189,11 @@ static int do_anonymous_page(struct vm_fault *vmf)
> if (vma->vm_flags & VM_WRITE)
> entry = pte_mkwrite(pte_mkdirty(entry));
>
> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> - &vmf->ptl);
> + if (!pte_map_lock(vmf)) {
> + mem_cgroup_cancel_charge(page, memcg, false);
> + put_page(page);
> + return VM_FAULT_RETRY;
> + }
> if (!pte_none(*vmf->pte))
> goto release;
>
This is more spaghetti, can the exit path be fixed up so we order things
consistently for all gotos?
> @@ -3294,8 +3317,9 @@ static int pte_alloc_one_map(struct vm_fault *vmf)
> * pte_none() under vmf->ptl protection when we return to
> * alloc_set_pte().
> */
> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> - &vmf->ptl);
> + if (!pte_map_lock(vmf))
> + return VM_FAULT_RETRY;
> +
> return 0;
> }
>
