Re: [PATCH v2 0/5] mm: migrate zbud pages

[Tomasz Stanislawski <t.stanislaws@samsung.com>]

On 09/25/2013 11:57 PM, Seth Jennings wrote:
> On Wed, Sep 25, 2013 at 07:09:50PM +0200, Tomasz Stanislawski wrote:
>>> I just had an idea this afternoon to potentially kill both these birds with one
>>> stone: Replace the rbtree in zswap with an address_space.
>>>
>>> Each swap type would have its own page_tree to organize the compressed objects
>>> by type and offset (radix tree is more suited for this anyway) and a_ops that
>>> could be called by shrink_page_list() (writepage) or the migration code
>>> (migratepage).
>>>
>>> Then zbud pages could be put on the normal LRU list, maybe at the beginning of
>>> the inactive LRU so they would live for another cycle through the list, then be
>>> reclaimed in the normal way with the mapping->a_ops->writepage() pointing to a
>>> zswap_writepage() function that would decompress the pages and call
>>> __swap_writepage() on them.
>>>
>>> This might actually do away with the explicit pool size too as the compressed
>>> pool pages wouldn't be outside the control of the MM anymore.
>>>
>>> I'm just starting to explore this but I think it has promise.
>>>
>>> Seth
>>>
>>
>> Hi Seth,
>> There is a problem with the proposed idea.
>> The radix tree used 'struct address_space' is a part of
>> a bigger data structure.
>> The radix tree is used to translate an offset to a page.
>> That is ok for zswap. But struct page has a field named 'index'.
>> The MM assumes that this index is an offset in radix tree
>> where one can find the page. A lot is done by MM to sustain
>> this consistency.
> 
> Yes, this is how it is for page cache pages.  However, the MM is able to
> work differently with anonymous pages.  In the case of an anonymous
> page, the mapping field points to an anon_vma struct, or, if ksm in
> enabled and dedup'ing the page, a private ksm tracking structure.  If
> the anonymous page is fully unmapped and resides only in the swap cache,
> the page mapping is NULL.  So there is precedent for the fields to mean
> other things.

Hi Seth,
You are right that page->mapping is NULL for pages in swap_cache but
page_mapping() is not NULL in such a case. The mapping is taken from
struct address_space swapper_spaces[]. It is still an address space,
and it should preserve constraints for struct address_space.
The same happen for page->index and page_index().

> 
> The question is how to mark and identify zbud pages among the other page
> types that will be on the LRU.  There are many ways.  The question is
> what is the best and most acceptable way.
> 

If you consider hacking I have some idea how address_space could utilized for ZBUD.
One solution whould be using tags in a radix tree. Every entry in a radix tree
can have a few bits assigned to it. Currently 3 bits are supported:

>From include/linux/fs.h
#define PAGECACHE_TAG_DIRTY  0
#define PAGECACHE_TAG_WRITEBACK      1
#define PAGECACHE_TAG_TOWRITE        2

You could add a new bit or utilize one of existing ones.

The other idea is use a trick from a RB trees and scatter-gather lists.
I mean using the last bits of pointers to keep some metadata.
Values of 'struct page *' variables are aligned to a pointer alignment which is
4 for 32-bit CPUs and 8 for 64-bit ones (not sure). This means that one could
could use the last bit of page pointer in a radix tree to track if a swap entry
refers to a lower or a higher part of a ZBUD page.
I think it is a serious hacking/obfuscation but it may work with the minimal
amount of changes to MM. Adding only (x&~3) while extracting page pointer is
probably enough.

What do you think about this idea?

>>
>> In case of zbud, there are two swap offset pointing to
>> the same page. There might be more if zsmalloc is used.
>> What is worse it is possible that one swap entry could
>> point to data that cross a page boundary.
> 
> We just won't set page->index since it doesn't have a good meaning in
> our case.  Swap cache pages also don't use index, although is seems to
> me that they could since there is a 1:1 mapping of a swap cache page to
> a swap offset and the index field isn't being used for anything else.
> But I digress...

OK.

> 
>>
>> Of course, one could try to modify MM to support
>> multiple mapping of a page in the radix tree.
>> But I think that MM guys will consider this as a hack
>> and they will not accept it.
> 
> Yes, it will require some changes to the MM to handle zbud pages on the
> LRU.  I'm thinking that it won't be too intrusive, depending on how we
> choose to mark zbud pages.
> 

Anyway, I think that zswap should use two index engines.
I mean index in Data Base meaning.
One index is used to translate swap_entry to compressed page.
And another one to be used by reclaim and migration by MM,
probably address_space is a best choice.
Zbud would responsible for keeping consistency
between mentioned indexes.

Regards,
Tomasz Stanislawski

> Seth
> 
>>
>> Regards,
>> Tomasz Stanislawski
>>
>>
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>>
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