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SubjectRe: [PATCH 1/6] mmput: use notifier chain to call subsystem exit handler.
On Fri, 27 Jun 2014, Jérôme Glisse wrote:

> From: Jérôme Glisse <jglisse@redhat.com>
>
> Several subsystem require a callback when a mm struct is being destroy
> so that they can cleanup there respective per mm struct. Instead of
> having each subsystem add its callback to mmput use a notifier chain
> to call each of the subsystem.
>
> This will allow new subsystem to register callback even if they are
> module. There should be no contention on the rw semaphore protecting
> the call chain and the impact on the code path should be low and
> burried in the noise.
>
> Note that this patch also move the call to cleanup functions after
> exit_mmap so that new call back can assume that mmu_notifier_release
> have already been call. This does not impact existing cleanup functions
> as they do not rely on anything that exit_mmap is freeing. Also moved
> khugepaged_exit to exit_mmap so that ordering is preserved for that
> function.
>
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> ---
> fs/aio.c | 29 ++++++++++++++++++++++-------
> include/linux/aio.h | 2 --
> include/linux/ksm.h | 11 -----------
> include/linux/sched.h | 5 +++++
> include/linux/uprobes.h | 1 -
> kernel/events/uprobes.c | 19 ++++++++++++++++---
> kernel/fork.c | 22 ++++++++++++++++++----
> mm/ksm.c | 26 +++++++++++++++++++++-----
> mm/mmap.c | 3 +++
> 9 files changed, 85 insertions(+), 33 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index c1d8c48..1d06e92 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -40,6 +40,7 @@
> #include <linux/ramfs.h>
> #include <linux/percpu-refcount.h>
> #include <linux/mount.h>
> +#include <linux/notifier.h>
>
> #include <asm/kmap_types.h>
> #include <asm/uaccess.h>
> @@ -774,20 +775,22 @@ ssize_t wait_on_sync_kiocb(struct kiocb *req)
> EXPORT_SYMBOL(wait_on_sync_kiocb);
>
> /*
> - * exit_aio: called when the last user of mm goes away. At this point, there is
> + * aio_exit: called when the last user of mm goes away. At this point, there is
> * no way for any new requests to be submited or any of the io_* syscalls to be
> * called on the context.
> *
> * There may be outstanding kiocbs, but free_ioctx() will explicitly wait on
> * them.
> */
> -void exit_aio(struct mm_struct *mm)
> +static int aio_exit(struct notifier_block *nb,
> + unsigned long action, void *data)
> {
> + struct mm_struct *mm = data;
> struct kioctx_table *table = rcu_dereference_raw(mm->ioctx_table);
> int i;
>
> if (!table)
> - return;
> + return 0;
>
> for (i = 0; i < table->nr; ++i) {
> struct kioctx *ctx = table->table[i];
> @@ -796,10 +799,10 @@ void exit_aio(struct mm_struct *mm)
> continue;
> /*
> * We don't need to bother with munmap() here - exit_mmap(mm)
> - * is coming and it'll unmap everything. And we simply can't,
> - * this is not necessarily our ->mm.
> - * Since kill_ioctx() uses non-zero ->mmap_size as indicator
> - * that it needs to unmap the area, just set it to 0.
> + * have already been call and everything is unmap by now. But
> + * to be safe set ->mmap_size to 0 since aio_free_ring() uses
> + * non-zero ->mmap_size as indicator that it needs to unmap the
> + * area.
> */

Actually, I think the original part of the comment about kill_ioctx
was accurate, but the new reference to aio_free_ring looks like a typo
(?). I'd write the entire comment as follows (I've dropped the leading
whitespace, for email):

/*
* We don't need to bother with munmap() here - exit_mmap(mm)
* has already been called and everything is unmapped by now.
* But to be safe, set ->mmap_size to 0 since kill_ioctx() uses a
* non-zero >mmap_size as an indicator that it needs to unmap the
* area.
*/


> ctx->mmap_size = 0;
> kill_ioctx(mm, ctx, NULL);
> @@ -807,6 +810,7 @@ void exit_aio(struct mm_struct *mm)
>
> RCU_INIT_POINTER(mm->ioctx_table, NULL);
> kfree(table);
> + return 0;
> }
>
> static void put_reqs_available(struct kioctx *ctx, unsigned nr)
> @@ -1629,3 +1633,14 @@ SYSCALL_DEFINE5(io_getevents, aio_context_t, ctx_id,
> }
> return ret;
> }
> +
> +static struct notifier_block aio_mmput_nb = {
> + .notifier_call = aio_exit,
> + .priority = 1,
> +};
> +
> +static int __init aio_init(void)
> +{
> + return mmput_register_notifier(&aio_mmput_nb);
> +}
> +subsys_initcall(aio_init);
> diff --git a/include/linux/aio.h b/include/linux/aio.h
> index d9c92da..6308fac 100644
> --- a/include/linux/aio.h
> +++ b/include/linux/aio.h
> @@ -73,7 +73,6 @@ static inline void init_sync_kiocb(struct kiocb *kiocb, struct file *filp)
> extern ssize_t wait_on_sync_kiocb(struct kiocb *iocb);
> extern void aio_complete(struct kiocb *iocb, long res, long res2);
> struct mm_struct;
> -extern void exit_aio(struct mm_struct *mm);
> extern long do_io_submit(aio_context_t ctx_id, long nr,
> struct iocb __user *__user *iocbpp, bool compat);
> void kiocb_set_cancel_fn(struct kiocb *req, kiocb_cancel_fn *cancel);
> @@ -81,7 +80,6 @@ void kiocb_set_cancel_fn(struct kiocb *req, kiocb_cancel_fn *cancel);
> static inline ssize_t wait_on_sync_kiocb(struct kiocb *iocb) { return 0; }
> static inline void aio_complete(struct kiocb *iocb, long res, long res2) { }
> struct mm_struct;
> -static inline void exit_aio(struct mm_struct *mm) { }
> static inline long do_io_submit(aio_context_t ctx_id, long nr,
> struct iocb __user * __user *iocbpp,
> bool compat) { return 0; }
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index 3be6bb1..84c184f 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -20,7 +20,6 @@ struct mem_cgroup;
> int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, int advice, unsigned long *vm_flags);
> int __ksm_enter(struct mm_struct *mm);
> -void __ksm_exit(struct mm_struct *mm);
>
> static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
> {
> @@ -29,12 +28,6 @@ static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
> return 0;
> }
>
> -static inline void ksm_exit(struct mm_struct *mm)
> -{
> - if (test_bit(MMF_VM_MERGEABLE, &mm->flags))
> - __ksm_exit(mm);
> -}
> -
> /*
> * A KSM page is one of those write-protected "shared pages" or "merged pages"
> * which KSM maps into multiple mms, wherever identical anonymous page content
> @@ -83,10 +76,6 @@ static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
> return 0;
> }
>
> -static inline void ksm_exit(struct mm_struct *mm)
> -{
> -}
> -
> static inline int PageKsm(struct page *page)
> {
> return 0;
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 322d4fc..428b3cf 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2384,6 +2384,11 @@ static inline void mmdrop(struct mm_struct * mm)
> __mmdrop(mm);
> }
>
> +/* mmput call list of notifier and subsystem/module can register
> + * new one through this call.
> + */
> +extern int mmput_register_notifier(struct notifier_block *nb);
> +extern int mmput_unregister_notifier(struct notifier_block *nb);
> /* mmput gets rid of the mappings and all user-space */
> extern void mmput(struct mm_struct *);
> /* Grab a reference to a task's mm, if it is not already going away */
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index 4f844c6..44e7267 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -120,7 +120,6 @@ extern int uprobe_pre_sstep_notifier(struct pt_regs *regs);
> extern void uprobe_notify_resume(struct pt_regs *regs);
> extern bool uprobe_deny_signal(void);
> extern bool arch_uprobe_skip_sstep(struct arch_uprobe *aup, struct pt_regs *regs);
> -extern void uprobe_clear_state(struct mm_struct *mm);
> extern int arch_uprobe_analyze_insn(struct arch_uprobe *aup, struct mm_struct *mm, unsigned long addr);
> extern int arch_uprobe_pre_xol(struct arch_uprobe *aup, struct pt_regs *regs);
> extern int arch_uprobe_post_xol(struct arch_uprobe *aup, struct pt_regs *regs);
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 46b7c31..32b04dc 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -37,6 +37,7 @@
> #include <linux/percpu-rwsem.h>
> #include <linux/task_work.h>
> #include <linux/shmem_fs.h>
> +#include <linux/notifier.h>
>
> #include <linux/uprobes.h>
>
> @@ -1220,16 +1221,19 @@ static struct xol_area *get_xol_area(void)
> /*
> * uprobe_clear_state - Free the area allocated for slots.
> */
> -void uprobe_clear_state(struct mm_struct *mm)
> +static int uprobe_clear_state(struct notifier_block *nb,
> + unsigned long action, void *data)
> {
> + struct mm_struct *mm = data;
> struct xol_area *area = mm->uprobes_state.xol_area;
>
> if (!area)
> - return;
> + return 0;
>
> put_page(area->page);
> kfree(area->bitmap);
> kfree(area);
> + return 0;
> }
>
> void uprobe_start_dup_mmap(void)
> @@ -1979,9 +1983,14 @@ static struct notifier_block uprobe_exception_nb = {
> .priority = INT_MAX-1, /* notified after kprobes, kgdb */
> };
>
> +static struct notifier_block uprobe_mmput_nb = {
> + .notifier_call = uprobe_clear_state,
> + .priority = 0,
> +};
> +
> static int __init init_uprobes(void)
> {
> - int i;
> + int i, err;
>
> for (i = 0; i < UPROBES_HASH_SZ; i++)
> mutex_init(&uprobes_mmap_mutex[i]);
> @@ -1989,6 +1998,10 @@ static int __init init_uprobes(void)
> if (percpu_init_rwsem(&dup_mmap_sem))
> return -ENOMEM;
>
> + err = mmput_register_notifier(&uprobe_mmput_nb);
> + if (err)
> + return err;
> +
> return register_die_notifier(&uprobe_exception_nb);
> }
> __initcall(init_uprobes);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index dd8864f..b448509 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -87,6 +87,8 @@
> #define CREATE_TRACE_POINTS
> #include <trace/events/task.h>
>
> +static BLOCKING_NOTIFIER_HEAD(mmput_notifier);
> +
> /*
> * Protected counters by write_lock_irq(&tasklist_lock)
> */
> @@ -623,6 +625,21 @@ void __mmdrop(struct mm_struct *mm)
> EXPORT_SYMBOL_GPL(__mmdrop);
>
> /*
> + * Register a notifier that will be call by mmput
> + */
> +int mmput_register_notifier(struct notifier_block *nb)
> +{
> + return blocking_notifier_chain_register(&mmput_notifier, nb);
> +}
> +EXPORT_SYMBOL_GPL(mmput_register_notifier);
> +
> +int mmput_unregister_notifier(struct notifier_block *nb)
> +{
> + return blocking_notifier_chain_unregister(&mmput_notifier, nb);
> +}
> +EXPORT_SYMBOL_GPL(mmput_unregister_notifier);
> +
> +/*
> * Decrement the use count and release all resources for an mm.
> */
> void mmput(struct mm_struct *mm)
> @@ -630,11 +647,8 @@ void mmput(struct mm_struct *mm)
> might_sleep();
>
> if (atomic_dec_and_test(&mm->mm_users)) {
> - uprobe_clear_state(mm);
> - exit_aio(mm);
> - ksm_exit(mm);
> - khugepaged_exit(mm); /* must run before exit_mmap */
> exit_mmap(mm);
> + blocking_notifier_call_chain(&mmput_notifier, 0, mm);
> set_mm_exe_file(mm, NULL);
> if (!list_empty(&mm->mmlist)) {
> spin_lock(&mmlist_lock);
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 346ddc9..cb1e976 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -37,6 +37,7 @@
> #include <linux/freezer.h>
> #include <linux/oom.h>
> #include <linux/numa.h>
> +#include <linux/notifier.h>
>
> #include <asm/tlbflush.h>
> #include "internal.h"
> @@ -1586,7 +1587,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
> ksm_scan.mm_slot = slot;
> spin_unlock(&ksm_mmlist_lock);
> /*
> - * Although we tested list_empty() above, a racing __ksm_exit
> + * Although we tested list_empty() above, a racing ksm_exit
> * of the last mm on the list may have removed it since then.
> */
> if (slot == &ksm_mm_head)
> @@ -1658,9 +1659,9 @@ next_mm:
> /*
> * We've completed a full scan of all vmas, holding mmap_sem
> * throughout, and found no VM_MERGEABLE: so do the same as
> - * __ksm_exit does to remove this mm from all our lists now.
> - * This applies either when cleaning up after __ksm_exit
> - * (but beware: we can reach here even before __ksm_exit),
> + * ksm_exit does to remove this mm from all our lists now.
> + * This applies either when cleaning up after ksm_exit
> + * (but beware: we can reach here even before ksm_exit),
> * or when all VM_MERGEABLE areas have been unmapped (and
> * mmap_sem then protects against race with MADV_MERGEABLE).
> */
> @@ -1821,11 +1822,16 @@ int __ksm_enter(struct mm_struct *mm)
> return 0;
> }
>
> -void __ksm_exit(struct mm_struct *mm)
> +static int ksm_exit(struct notifier_block *nb,
> + unsigned long action, void *data)
> {
> + struct mm_struct *mm = data;
> struct mm_slot *mm_slot;
> int easy_to_free = 0;
>
> + if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
> + return 0;
> +
> /*
> * This process is exiting: if it's straightforward (as is the
> * case when ksmd was never running), free mm_slot immediately.
> @@ -1857,6 +1863,7 @@ void __ksm_exit(struct mm_struct *mm)
> down_write(&mm->mmap_sem);
> up_write(&mm->mmap_sem);
> }
> + return 0;
> }
>
> struct page *ksm_might_need_to_copy(struct page *page,
> @@ -2305,11 +2312,20 @@ static struct attribute_group ksm_attr_group = {
> };
> #endif /* CONFIG_SYSFS */
>
> +static struct notifier_block ksm_mmput_nb = {
> + .notifier_call = ksm_exit,
> + .priority = 2,
> +};
> +
> static int __init ksm_init(void)
> {
> struct task_struct *ksm_thread;
> int err;
>
> + err = mmput_register_notifier(&ksm_mmput_nb);
> + if (err)
> + return err;
> +

In order to be perfectly consistent with this routine's existing code, you
would want to write:

if (err)
goto out;

...but it does the same thing as your code. It' just a consistency thing.

> err = ksm_slab_init();
> if (err)
> goto out;
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 61aec93..b684a21 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2775,6 +2775,9 @@ void exit_mmap(struct mm_struct *mm)
> struct vm_area_struct *vma;
> unsigned long nr_accounted = 0;
>
> + /* Important to call this first. */
> + khugepaged_exit(mm);
> +
> /* mm's last user has gone, and its about to be pulled down */
> mmu_notifier_release(mm);
>
> --
> 1.9.0
>
> --
> 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: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>

Above points are extremely minor, so:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
John H.
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