Re: [PATCH] mm: Cleanup - Reorganize the shrink_page_list code into smaller functions

Tim Chen
2016-06-07T20:43:41Z

On Tue, 2016-06-07 at 17:21 +0900, Minchan Kim wrote:
> On Wed, Jun 01, 2016 at 11:23:53AM -0700, Tim Chen wrote:
> >
> > On Wed, 2016-06-01 at 16:12 +0900, Minchan Kim wrote:
> > >
> > >
> > > Hi Tim,
> > >
> > > To me, this reorganization is too limited and not good for me,
> > > frankly speaking. It works for only your goal which allocate batch
> > > swap slot, I guess. :)
> > >
> > > My goal is to make them work with batch page_check_references,
> > > batch try_to_unmap and batch __remove_mapping where we can avoid frequent
> > > mapping->lock(e.g., anon_vma or i_mmap_lock with hoping such batch locking
> > > help system performance) if batch pages has same inode or anon.
> > This is also my goal to group pages that are either under the same
> > mapping or are anonymous pages together so we can reduce the i_mmap_lock
> > acquisition.  One logic that's yet to be implemented in your patch
> > is the grouping of similar pages together so we only need one i_mmap_lock
> > acquisition.  Doing this efficiently is non-trivial.
> Hmm, my assumption is based on same inode pages are likely to order
> in LRU so no need to group them. If successive page in page_list comes
> from different inode, we can drop the lock and get new lock from new
> inode. That sounds strange?
>

Sounds reasonable. But your process function passed to spl_batch_pages may
need to be modified to know if the radix tree lock or swap info lock
has already been held, as it deals with only 1 page.  It may be
tricky as the lock may get acquired and dropped more than once in process
function.

Are you planning to update the patch with lock batching?

Thanks.

Tim

> >
> >
> > I punted the problem somewhat in my patch and elected to defer the processing
> > of the anonymous pages at the end so they are naturally grouped without
> > having to traverse the page_list more than once.  So I'm batching the
> > anonymous pages but the file mapped pages were not grouped.
> >
> > In your implementation, you may need to traverse the page_list in two pass, where the
> > first one is to categorize the pages and grouping them and the second one
> > is the actual processing.  Then the lock batching can be implemented
> > for the pages.  Otherwise the locking is still done page by page in
> > your patch, and can only be batched if the next page on page_list happens
> > to have the same mapping.  Your idea of using a spl_batch_pages is pretty
> Yes. as I said above, I expect pages in LRU would be likely to order per
> inode normally. If it's not, yeb, we need grouping but such overhead would
> mitigate the benefit of lock batch as SWAP_CLUSTER_MAX get bigger.
>
> >
> > neat.  It may need some enhancement so it is known whether some locks
> > are already held for lock batching purpose.
> >
> >
> > Thanks.
> >
> > Tim