One weakness which was introduced when the buffer LRU went away was
that GFP_NOFS allocations became equivalent to GFP_NOIO.  Because all
writeback goes via writepage/writepages, which requires entry into the
filesystem.

However now that swapout no longer calls bmap(), we can honour
GFP_NOFS's intent for swapcache pages.  So if the allocation request
specifies __GFP_IO and !__GFP_FS, we can wait on swapcache pages and we
can perform swapcache writeout.

This should strengthen the VM somewhat.