Re: [PATCH v6v3 02/12] mm: migrate: support non-lru movable page migration

On 06/16/2016 05:56 AM, Minchan Kim wrote:

On Wed, Jun 15, 2016 at 12:15:04PM +0530, Anshuman Khandual wrote:

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On 06/15/2016 08:02 AM, Minchan Kim wrote:

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Hi,

On Mon, Jun 13, 2016 at 03:08:19PM +0530, Anshuman Khandual wrote:

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On 05/31/2016 05:31 AM, Minchan Kim wrote:

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@@ -791,6 +921,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 	int rc = -EAGAIN;
 	int page_was_mapped = 0;
 	struct anon_vma *anon_vma = NULL;
+	bool is_lru = !__PageMovable(page);
 
 	if (!trylock_page(page)) {
 		if (!force || mode == MIGRATE_ASYNC)

@@ -871,6 +1002,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		goto out_unlock_both;
 	}
 
+	if (unlikely(!is_lru)) {
+		rc = move_to_new_page(newpage, page, mode);
+		goto out_unlock_both;
+	}
+

Hello Minchan,

I might be missing something here but does this implementation support the
scenario where these non LRU pages owned by the driver mapped as PTE into
process page table ? Because the "goto out_unlock_both" statement above
skips all the PTE unmap, putting a migration PTE and removing the migration
PTE steps.

You're right. Unfortunately, it doesn't support right now but surely,
it's my TODO after landing this work.

Could you share your usecase?

Sure.

Thanks a lot!

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My driver has privately managed non LRU pages which gets mapped into user space
process page table through f_ops->mmap() and vmops->fault() which then updates
the file RMAP (page->mapping->i_mmap) through page_add_file_rmap(page). One thing

Hmm, page_add_file_rmap is not exported function. How does your driver can use it?

Its not using the function directly, I just re-iterated the sequence of functions
above. (do_set_pte -> page_add_file_rmap) gets called after we grab the page from
driver through (__do_fault->vma->vm_ops->fault()).

Do you use vm_insert_pfn?
What type your vma is? VM_PFNMMAP or VM_MIXEDMAP?

I dont use vm_insert_pfn(). Here is the sequence of events how the user space
VMA gets the non LRU pages from the driver.

- Driver registers a character device with 'struct file_operations' binding
- Then the 'fops->mmap()' just binds the incoming 'struct vma' with a 'struct
  vm_operations_struct' which provides the 'vmops->fault()' routine which
  basically traps all page faults on the VMA and provides one page at a time
  through a driver specific allocation routine which hands over non LRU pages

The VMA is not anything special as such. Its what we get when we try to do a
simple mmap() on a file descriptor pointing to a character device. I can
figure out all the VM_* flags it holds after creation.

I want to make dummy driver to simulate your case.

Sure. I hope the above mentioned steps will help you but in case you need more
information, please do let me know.

It would be very helpful to implement/test pte-mapped non-lru page
migration feature. That's why I ask now.

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to note here is that the page->mapping eventually points to struct address_space
(file->f_mapping) which belongs to the character device file (created using mknod)
which we are using for establishing the mmap() regions in the user space.

Now as per this new framework, all the page's are to be made __SetPageMovable before
passing the list down to migrate_pages(). Now __SetPageMovable() takes *new* struct
address_space as an argument and replaces the existing page->mapping. Now thats the
problem, we have lost all our connection to the existing file RMAP information. This

We could change __SetPageMovable doesn't need mapping argument.
Instead, it just marks PAGE_MAPPING_MOVABLE into page->mapping.
For that, user should take care of setting page->mapping earlier than
marking the flag.

Sounds like a good idea, that way we dont loose the reverse mapping information.

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stands as a problem when we try to migrate these non LRU pages which are PTE mapped.
The rmap_walk_file() never finds them in the VMA, skips all the migrate PTE steps and
then the migration eventually fails.

Seems like assigning a new struct address_space to the page through __SetPageMovable()
is the source of the problem. Can it take the existing (file->f_mapping) as an argument

We can set existing file->f_mapping under the page_lock.

Thats another option along with what you mentioned above.

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in there ? Sure, but then can we override file system generic ->isolate(), ->putback(),

I don't get it. Why does it override file system generic functions?

Sure it does not, it was just an wild idea to over come the problem.