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

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!

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

I want to make dummy driver to simulate your case. It would be very helpful to implement/test pte-mapped non-lru page migration feature. That's why I ask now.

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.

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.

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?

->migratepages() functions ? I dont think so. I am sure, there must be some work around to fix this problem for the driver. But we need to rethink this framework from supporting these mapped non LRU pages point of view.

I might be missing something here, feel free to point out.

• Anshuman