Re: [GIT PULL] aio: changes for 3.19

Andrew Morton
2014-12-14T22:13:35Z

On Sun, 14 Dec 2014 16:52:21 -0500 Benjamin LaHaise <bcrl@kvack.org> wrote:

> On Sun, Dec 14, 2014 at 01:47:32PM -0800, Linus Torvalds wrote:
> > On Sun, Dec 14, 2014 at 12:22 PM, Benjamin LaHaise <bcrl@kvack.org> wrote:
> > >
> > > Pavel Emelyanov (1):
> > > aio: Make it possible to remap aio ring
> >
> > So quite frankly, I think this should have had more acks from VM
> > people. The patch looks ok to me, but it took me by surprise, and I
> > don't see much any discussion about it on linux-mm either..
>
> Sadly, nobody responded. Maybe akpm can chime in on this change (included
> below for ease of review and akpm added to the To:)?
>
> -ben
> --
> "Thought is the essence of where you are now."
>
> commit e4a0d3e720e7e508749c1439b5ba3aff56c92976
> Author: Pavel Emelyanov <xemul@parallels.com>
> Date: Thu Sep 18 19:56:17 2014 +0400
>
> aio: Make it possible to remap aio ring

The patch appears to be a bugfix which coincidentally helps CRIU?

If it weren't for the bugfix part, I'd be asking "why not pass the
desired virtual address into io_setup()?".

> There are actually two issues this patch addresses. Let me start with
> the one I tried to solve in the beginning.
>
> So, in the checkpoint-restore project (criu) we try to dump tasks'
> state and restore one back exactly as it was. One of the tasks' state
> bits is rings set up with io_setup() call. There's (almost) no problems
> in dumping them, there's a problem restoring them -- if I dump a task
> with aio ring originally mapped at address A, I want to restore one
> back at exactly the same address A. Unfortunately, the io_setup() does
> not allow for that -- it mmaps the ring at whatever place mm finds
> appropriate (it calls do_mmap_pgoff() with zero address and without
> the MAP_FIXED flag).
>
> To make restore possible I'm going to mremap() the freshly created ring
> into the address A (under which it was seen before dump). The problem is
> that the ring's virtual address is passed back to the user-space as the
> context ID and this ID is then used as search key by all the other io_foo()
> calls. Reworking this ID to be just some integer doesn't seem to work, as
> this value is already used by libaio as a pointer using which this library
> accesses memory for aio meta-data.
>
> So, to make restore work we need to make sure that
>
> a) ring is mapped at desired virtual address
> b) kioctx->user_id matches this value
>
> Having said that, the patch makes mremap() on aio region update the
> kioctx's user_id and mmap_base values.
>
> Here appears the 2nd issue I mentioned in the beginning of this mail.
> If (regardless of the C/R dances I do) someone creates an io context
> with io_setup(), then mremap()-s the ring and then destroys the context,
> the kill_ioctx() routine will call munmap() on wrong (old) address.
> This will result in a) aio ring remaining in memory and b) some other
> vma get unexpectedly unmapped.
>
> What do you think?
>
> ...
>
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -286,12 +286,37 @@ static void aio_free_ring(struct kioctx *ctx)
>
> static int aio_ring_mmap(struct file *file, struct vm_area_struct *vma)
> {
> + vma->vm_flags |= VM_DONTEXPAND;

I don't think this was changelogged? A comment here would be best.

> vma->vm_ops = &generic_file_vm_ops;
> return 0;
> }
>
> +static void aio_ring_remap(struct file *file, struct vm_area_struct *vma)
> +{
> + struct mm_struct *mm = vma->vm_mm;
> + struct kioctx_table *table;
> + int i;
> +
> + spin_lock(&mm->ioctx_lock);
> + rcu_read_lock();
> + table = rcu_dereference(mm->ioctx_table);
> + for (i = 0; i < table->nr; i++) {
> + struct kioctx *ctx;
> +
> + ctx = table->table[i];
> + if (ctx && ctx->aio_ring_file == file) {
> + ctx->user_id = ctx->mmap_base = vma->vm_start;
> + break;
> + }
> + }
> +
> + rcu_read_unlock();
> + spin_unlock(&mm->ioctx_lock);
> +}

Looks simple enough. Possibly so simple that it doesn't need commenting ;)

> static const struct file_operations aio_ring_fops = {
> .mmap = aio_ring_mmap,
> + .mremap = aio_ring_remap,
> };
>
> #if IS_ENABLED(CONFIG_MIGRATION)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 9ab779e..85f378c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1497,6 +1497,7 @@ struct file_operations {
> long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
> long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
> int (*mmap) (struct file *, struct vm_area_struct *);
> + void (*mremap)(struct file *, struct vm_area_struct *);

Documentation/filesystems/vfs.txt needs updating. Note that it says
"all methods are called without any locks being held, unless otherwise
noted", but ->mremap() is called under mmap_sem (at least) so otherwise
noting will be needed.

It might make sense for ->mremap() to return an errno, but we can
change that later if needed.

> int (*open) (struct inode *, struct file *);
> int (*flush) (struct file *, fl_owner_t id);
> int (*release) (struct inode *, struct file *);
> diff --git a/mm/mremap.c b/mm/mremap.c
> index b147f66..c855922 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -288,7 +288,8 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> old_len = new_len;
> old_addr = new_addr;
> new_addr = -ENOMEM;
> - }
> + } else if (vma->vm_file && vma->vm_file->f_op->mremap)
> + vma->vm_file->f_op->mremap(vma->vm_file, new_vma);
>

The patch overall is a no-op from an MM perspective and seems OK to me.