linux-kernel - Re: [GIT PULL] aio: changes for 3.19

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Date:	Sun, 14 Dec 2014 17:39:36 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-aio@...ck.org, Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [GIT PULL] aio: changes for 3.19

On Sun, Dec 14, 2014 at 02:13:36PM -0800, Andrew Morton wrote:
> The patch appears to be a bugfix which coincidentally helps CRIU?

Yes.

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

It's entirely possible someone might have a need to mremap the event ring, 
but nobody seems to have tried until now.

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

How about the documentation/comment updates below?

I'll try to be more aggressive about getting signoffs on these wider changes 
in the future.

		-ben
-- 
"Thought is the essence of where you are now."

aio/mm: update documentation for mremap hook in commit e4a0d3e720e7e508749c1439b5ba3aff56c92976

Add a few more comments and documentation to explain the mremap hook introduced
in commit e4a0d3e720e7e508749c1439b5ba3aff56c92976.

Signed-off-by: Benjamin LaHaise <bcrl@...ck.org>

 Documentation/filesystems/vfs.txt |    4 ++++
 fs/aio.c                          |    8 ++++++++
 2 files changed, 12 insertions(+)

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 43ce050..a9f3df5 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -820,6 +820,7 @@ 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 *);
 	int (*open) (struct inode *, struct file *);
 	int (*flush) (struct file *);
 	int (*release) (struct inode *, struct file *);
@@ -868,6 +869,9 @@ otherwise noted.
 
   mmap: called by the mmap(2) system call
 
+  mremap: called by the mremap(2) system call.  Called holding mmap_sem for
+	write.
+
   open: called by the VFS when an inode should be opened. When the VFS
 	opens a file, it creates a new "struct file". It then calls the
 	open method for the newly allocated file structure. You might
diff --git a/fs/aio.c b/fs/aio.c
index 1b7893e..aba5385 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -286,11 +286,19 @@ static void aio_free_ring(struct kioctx *ctx)
 
 static int aio_ring_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	/* Resizing the event ring is not supported; mark it so. */
 	vma->vm_flags |= VM_DONTEXPAND;
 	vma->vm_ops = &generic_file_vm_ops;
 	return 0;
 }
 
+/* aio_ring_remap()
+ *	Called when th aio event ring is being relocated within the process'
+ *	address space.  The primary purpose is to update the saved address of
+ *	the aio event ring so that when the ioctx is detroyed, it gets removed
+ *	from the correct userspace address.  This is typically used when
+ *	reloading a process back into memory by checkpoint-restore.
+ */
 static void aio_ring_remap(struct file *file, struct vm_area_struct *vma)
 {
 	struct mm_struct *mm = vma->vm_mm;
--
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