On 14/12/2017 22:30, David Rientjes wrote:
> Commit 4d4bbd8526a8 ("mm, oom_reaper: skip mm structs with mmu notifiers")
> prevented the oom reaper from unmapping private anonymous memory with the
> oom reaper when the oom victim mm had mmu notifiers registered.
>
> The rationale is that doing mmu_notifier_invalidate_range_{start,end}()
> around the unmap_page_range(), which is needed, can block and the oom
> killer will stall forever waiting for the victim to exit, which may not
> be possible without reaping.
>
> That concern is real, but only true for mmu notifiers that have blockable
> invalidate_range_{start,end}() callbacks. This patch adds a "flags" field
> to mmu notifier ops that can set a bit to indicate that these callbacks do
> not block.
>
> The implementation is steered toward an expensive slowpath, such as after
> the oom reaper has grabbed mm->mmap_sem of a still alive oom victim.
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> v2:
> - specifically exclude mmu_notifiers without invalidate callbacks
> - move flags to mmu_notifier_ops per Paolo
> - reverse flag from blockable -> not blockable per Christian
>
> drivers/infiniband/hw/hfi1/mmu_rb.c | 1 +
> drivers/iommu/amd_iommu_v2.c | 1 +
> drivers/iommu/intel-svm.c | 1 +
> drivers/misc/sgi-gru/grutlbpurge.c | 1 +
> include/linux/mmu_notifier.h | 21 +++++++++++++++++++++
> mm/mmu_notifier.c | 31 +++++++++++++++++++++++++++++++
> virt/kvm/kvm_main.c | 1 +
> 7 files changed, 57 insertions(+)
>
> diff --git a/drivers/infiniband/hw/hfi1/mmu_rb.c b/drivers/infiniband/hw/hfi1/mmu_rb.c
> --- a/drivers/infiniband/hw/hfi1/mmu_rb.c
> +++ b/drivers/infiniband/hw/hfi1/mmu_rb.c
> @@ -77,6 +77,7 @@ static void do_remove(struct mmu_rb_handler *handler,
> static void handle_remove(struct work_struct *work);
>
> static const struct mmu_notifier_ops mn_opts = {
> + .flags = MMU_INVALIDATE_DOES_NOT_BLOCK,
> .invalidate_range_start = mmu_notifier_range_start,
> };
>
> diff --git a/drivers/iommu/amd_iommu_v2.c b/drivers/iommu/amd_iommu_v2.c
> --- a/drivers/iommu/amd_iommu_v2.c
> +++ b/drivers/iommu/amd_iommu_v2.c
> @@ -427,6 +427,7 @@ static void mn_release(struct mmu_notifier *mn, struct mm_struct *mm)
> }
>
> static const struct mmu_notifier_ops iommu_mn = {
> + .flags = MMU_INVALIDATE_DOES_NOT_BLOCK,
> .release = mn_release,
> .clear_flush_young = mn_clear_flush_young,
> .invalidate_range = mn_invalidate_range,
> diff --git a/drivers/iommu/intel-svm.c b/drivers/iommu/intel-svm.c
> --- a/drivers/iommu/intel-svm.c
> +++ b/drivers/iommu/intel-svm.c
> @@ -276,6 +276,7 @@ static void intel_mm_release(struct mmu_notifier *mn, struct mm_struct *mm)
> }
>
> static const struct mmu_notifier_ops intel_mmuops = {
> + .flags = MMU_INVALIDATE_DOES_NOT_BLOCK,
> .release = intel_mm_release,
> .change_pte = intel_change_pte,
> .invalidate_range = intel_invalidate_range,
> diff --git a/drivers/misc/sgi-gru/grutlbpurge.c b/drivers/misc/sgi-gru/grutlbpurge.c
> --- a/drivers/misc/sgi-gru/grutlbpurge.c
> +++ b/drivers/misc/sgi-gru/grutlbpurge.c
> @@ -258,6 +258,7 @@ static void gru_release(struct mmu_notifier *mn, struct mm_struct *mm)
>
>
> static const struct mmu_notifier_ops gru_mmuops = {
> + .flags = MMU_INVALIDATE_DOES_NOT_BLOCK,
> .invalidate_range_start = gru_invalidate_range_start,
> .invalidate_range_end = gru_invalidate_range_end,
> .release = gru_release,
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -10,6 +10,9 @@
> struct mmu_notifier;
> struct mmu_notifier_ops;
>
> +/* mmu_notifier_ops flags */
> +#define MMU_INVALIDATE_DOES_NOT_BLOCK (0x01)
> +
> #ifdef CONFIG_MMU_NOTIFIER
>
> /*
> @@ -26,6 +29,15 @@ struct mmu_notifier_mm {
> };
>
> struct mmu_notifier_ops {
> + /*
> + * Flags to specify behavior of callbacks for this MMU notifier.
> + * Used to determine which context an operation may be called.
> + *
> + * MMU_INVALIDATE_DOES_NOT_BLOCK: invalidate_{start,end} does not
> + * block
> + */
> + int flags;
> +
> /*
> * Called either by mmu_notifier_unregister or when the mm is
> * being destroyed by exit_mmap, always before all pages are
> @@ -137,6 +149,9 @@ struct mmu_notifier_ops {
> * page. Pages will no longer be referenced by the linux
> * address space but may still be referenced by sptes until
> * the last refcount is dropped.
> + *
> + * If both of these callbacks cannot block, mmu_notifier_ops.flags
> + * should have MMU_INVALIDATE_DOES_NOT_BLOCK set.
> */
> void (*invalidate_range_start)(struct mmu_notifier *mn,
> struct mm_struct *mm,
> @@ -218,6 +233,7 @@ extern void __mmu_notifier_invalidate_range_end(struct mm_struct *mm,
> bool only_end);
> extern void __mmu_notifier_invalidate_range(struct mm_struct *mm,
> unsigned long start, unsigned long end);
> +extern int mm_has_blockable_invalidate_notifiers(struct mm_struct *mm);
>
> static inline void mmu_notifier_release(struct mm_struct *mm)
> {
> @@ -457,6 +473,11 @@ static inline void mmu_notifier_invalidate_range(struct mm_struct *mm,
> {
> }
>
> +static inline int mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
> +{
> + return 0;
> +}
> +
> static inline void mmu_notifier_mm_init(struct mm_struct *mm)
> {
> }
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -236,6 +236,37 @@ void __mmu_notifier_invalidate_range(struct mm_struct *mm,
> }
> EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range);
>
> +/*
> + * Must be called while holding mm->mmap_sem for either read or write.
> + * The result is guaranteed to be valid until mm->mmap_sem is dropped.
> + */
> +int mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
> +{
> + struct mmu_notifier *mn;
> + int id;
> + int ret = 0;
> +
> + WARN_ON_ONCE(down_write_trylock(&mm->mmap_sem));
> +
> + if (!mm_has_notifiers(mm))
> + return ret;
> +
> + id = srcu_read_lock(&srcu);
> + hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
> + if (!mn->ops->invalidate_range &&
> + !mn->ops->invalidate_range_start &&
> + !mn->ops->invalidate_range_end)
> + continue;
> +
> + if (!(mn->ops->flags & MMU_INVALIDATE_DOES_NOT_BLOCK)) {
> + ret = 1;
> + break;
> + }
> + }
> + srcu_read_unlock(&srcu, id);
> + return ret;
> +}
> +
> static int do_mmu_notifier_register(struct mmu_notifier *mn,
> struct mm_struct *mm,
> int take_mmap_sem)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -476,6 +476,7 @@ static void kvm_mmu_notifier_release(struct mmu_notifier *mn,
> }
>
> static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
> + .flags = MMU_INVALIDATE_DOES_NOT_BLOCK,
> .invalidate_range_start = kvm_mmu_notifier_invalidate_range_start,
> .invalidate_range_end = kvm_mmu_notifier_invalidate_range_end,
> .clear_flush_young = kvm_mmu_notifier_clear_flush_young,
>
Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
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