Linux Virtualization - Jun 2016 - [PATCH v6v3 02/12] mm: migrate: support non-lru movable page migration

On 06/15/2016 08:02 AM, Minchan Kim wrote:
> Hi,
> 
> On Mon, Jun 13, 2016 at 03:08:19PM +0530, Anshuman Khandual wrote:
>> > On 05/31/2016 05:31 AM, Minchan Kim wrote:
>>> > > @@ -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.

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

On Wed, Jun 15, 2016 at 12:15:04PM +0530, Anshuman Khandual
wrote:
> On 06/15/2016 08:02 AM, Minchan Kim wrote:
> > Hi,
> > 
> > On Mon, Jun 13, 2016 at 03:08:19PM +0530, Anshuman Khandual wrote:
> >> > On 05/31/2016 05:31 AM, Minchan Kim wrote:
> >>> > > @@ -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
>

On 06/16/2016 05:56 AM, Minchan Kim wrote:
> On Wed, Jun 15, 2016 at 12:15:04PM +0530, Anshuman Khandual wrote:
>> On 06/15/2016 08:02 AM, Minchan Kim wrote:
>>> Hi,
>>>
>>> On Mon, Jun 13, 2016 at 03:08:19PM +0530, Anshuman Khandual wrote:
>>>>> On 05/31/2016 05:31 AM, Minchan Kim wrote:
>>>>>>> @@ -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?
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.
> 
>> 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.
> 
>> 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.
> 
>> 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.