[RFC] TTM shrinking revisited

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Hi, Christian, others.

I'm starting to take a look at the TTM shrinker again. We'll probably be
needing it at least for supporting integrated hardware with the xe driver.

So assuming that the last attempt failed because of the need to allocate
shmem pages and lack of writeback at shrink time, I was thinking of the
following approach: (A rough design sketch of the core support for the
last bullet is in patch 1/1. It of course needs polishing if the interface
is at all accepted by the mm people).

Before embarking on this, any feedback or comments would be greatly
appreciated:

*) Avoid TTM swapping when no swap space is available. Better to adjust the
   TTM swapout watermark, as no pages can be freed to the system anyway.
*) Complement the TTM swapout watermark with a shrinker.
   For cached pages, that may hopefully remove the need for the watermark.
   Possibly a watermark needs to remain for wc pages and / or dma pages,
   depending on how well shrinking them works.
*) Trigger immediate writeback of pages handed to the swapcache / shmem,
   at least when the shrinker is called from kswapd.
*) Hide ttm_tt_swap[out|in] details in the ttm_pool code. In the pool code
   we have more details about the backing pages and can split pages,
   transition caching state and copy as necessary. Also investigate the
   possibility of reusing pool pages in a smart way if copying is needed.
*) See if we can directly insert pages into the swap-cache instead of
   taking the shmem detour, something along with the attached patch 1 RFC.

Thanks,
Thomas