Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Oct 27, 2021 at 10:35 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Oct 27, 2021 at 09:08:21AM -0700, Suren Baghdasaryan wrote:
> > Unconditional mmap_write_lock around free_pgtables in exit_mmap seems
> > to me the most semantically correct way forward and the pushback is on
> > the basis of regressing performance of the exit path. I would like to
> > measure that regression to confirm this. I don't have access to a big
> > machine but will ask someone in another Google team to try the test
> > Michal wrote here
> > https://lore.kernel.org/all/20170725142626.GJ26723@dhcp22.suse.cz/ on
> > a server with and without a custom patch.
>
> Sorry to hijack this, but could you ask that team to also test this
> patch?  I think there's probably a good-sized win here, but I have no
> profiles to share at this point.  I've only done light testing, and
> it may have bugs.
>
> NB: I only did the exit() path here.  fork() conversion is left as an
> exercise for the reader^W^W Liam.

To clarify, this patch does not change the mmap_write_lock portion of
exit_mmap. Do you want to test it in isolation or with the locking
changes in exit_mmap I mentioned?

>
> From 5f9daa14a5e58c86a73eccf59abe23d131004926 Mon Sep 17 00:00:00 2001
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Date: Wed, 27 Oct 2021 13:28:35 -0400
> Subject: [PATCH] mm: Add vmavec
>
> The vmavec lets us allocate and free batches of VMAs instead of
> one at a time.  Should improve fork() and exit() performance.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/vmavec.h | 38 ++++++++++++++++++++++++++++++++++++++
>  kernel/fork.c          | 17 +++++++++++++++++
>  mm/mmap.c              | 30 ++++++++++++++++++++++--------
>  3 files changed, 77 insertions(+), 8 deletions(-)
>  create mode 100644 include/linux/vmavec.h
>
> diff --git a/include/linux/vmavec.h b/include/linux/vmavec.h
> new file mode 100644
> index 000000000000..8a324e2e1258
> --- /dev/null
> +++ b/include/linux/vmavec.h
> @@ -0,0 +1,38 @@
> +/*
> + * A vma vector is an array of vm_area_structs, with a counter.
> + */
> +
> +struct vm_area_struct;
> +
> +#define VMAVEC_SIZE    15
> +
> +struct vmavec {
> +       unsigned char nr;
> +       void *vmas[VMAVEC_SIZE];
> +};
> +
> +#define VMAVEC(name)   struct vmavec name = { }
> +
> +static inline bool vmavec_full(struct vmavec *vmavec)
> +{
> +       return vmavec->nr == VMAVEC_SIZE;
> +}
> +
> +static inline bool vmavec_empty(struct vmavec *vmavec)
> +{
> +       return vmavec->nr == 0;
> +}
> +
> +static inline
> +void vmavec_push(struct vmavec *vmavec, struct vm_area_struct *vma)
> +{
> +       vmavec->vmas[vmavec->nr++] = vma;
> +}
> +
> +static inline struct vm_area_struct *vmavec_pop(struct vmavec *vmavec)
> +{
> +       return vmavec->vmas[--vmavec->nr];
> +}
> +
> +void vm_area_free_vec(struct vmavec *vmavec);
> +void vm_area_alloc_vec(struct mm_struct *mm, struct vmavec *vmavec);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 38681ad44c76..ea7e8bd00be8 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -97,6 +97,7 @@
>  #include <linux/scs.h>
>  #include <linux/io_uring.h>
>  #include <linux/bpf.h>
> +#include <linux/vmavec.h>
>
>  #include <asm/pgalloc.h>
>  #include <linux/uaccess.h>
> @@ -375,6 +376,22 @@ void vm_area_free(struct vm_area_struct *vma)
>         kmem_cache_free(vm_area_cachep, vma);
>  }
>
> +void vm_area_alloc_vec(struct mm_struct *mm, struct vmavec *vmavec)
> +{
> +       int i;
> +
> +       vmavec->nr = kmem_cache_alloc_bulk(vm_area_cachep, GFP_KERNEL,
> +                               VMAVEC_SIZE, vmavec->vmas);
> +       for (i = 0; i < vmavec->nr; i++)
> +               vma_init(vmavec->vmas[i], mm);
> +}
> +
> +void vm_area_free_vec(struct vmavec *vmavec)
> +{
> +       kmem_cache_free_bulk(vm_area_cachep, vmavec->nr, vmavec->vmas);
> +       vmavec->nr = 0;
> +}
> +
>  static void account_kernel_stack(struct task_struct *tsk, int account)
>  {
>         void *stack = task_stack_page(tsk);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 88dcc5c25225..bff4e94eec8c 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -47,6 +47,7 @@
>  #include <linux/pkeys.h>
>  #include <linux/oom.h>
>  #include <linux/sched/mm.h>
> +#include <linux/vmavec.h>
>
>  #include <linux/uaccess.h>
>  #include <asm/cacheflush.h>
> @@ -172,19 +173,24 @@ void unlink_file_vma(struct vm_area_struct *vma)
>         }
>  }
>
> -/*
> - * Close a vm structure and free it, returning the next.
> - */
> -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
> +static void __remove_vma(struct vm_area_struct *vma)
>  {
> -       struct vm_area_struct *next = vma->vm_next;
> -
>         might_sleep();
>         if (vma->vm_ops && vma->vm_ops->close)
>                 vma->vm_ops->close(vma);
>         if (vma->vm_file)
>                 fput(vma->vm_file);
>         mpol_put(vma_policy(vma));
> +}
> +
> +/*
> + * Close a vm structure and free it, returning the next.
> + */
> +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
> +{
> +       struct vm_area_struct *next = vma->vm_next;
> +
> +       __remove_vma(vma);
>         vm_area_free(vma);
>         return next;
>  }
> @@ -3125,6 +3131,7 @@ void exit_mmap(struct mm_struct *mm)
>  {
>         struct mmu_gather tlb;
>         struct vm_area_struct *vma;
> +       VMAVEC(vmavec);
>         unsigned long nr_accounted = 0;
>
>         /* mm's last user has gone, and its about to be pulled down */
> @@ -3179,9 +3186,16 @@ void exit_mmap(struct mm_struct *mm)
>         while (vma) {
>                 if (vma->vm_flags & VM_ACCOUNT)
>                         nr_accounted += vma_pages(vma);
> -               vma = remove_vma(vma);
> -               cond_resched();
> +               __remove_vma(vma);
> +               vmavec_push(&vmavec, vma);
> +               vma = vma->vm_next;
> +               if (vmavec_full(&vmavec)) {
> +                       vm_area_free_vec(&vmavec);
> +                       cond_resched();
> +               }
>         }
> +       if (!vmavec_empty(&vmavec))
> +               vm_area_free_vec(&vmavec);
>         vm_unacct_memory(nr_accounted);
>  }
>
> --
> 2.33.0
>