Hi, Christian,
Thanks for the feedback. Some additional inline comments and questions:
On 1/4/23 11:31, Christian König wrote:
Am 30.12.22 um 12:11 schrieb Thomas Hellström:
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.
Yeah, that's what I've already tried and failed miserable exactly
because of what you described above.
Do you have a test-case for this or a typical failing scenario I can
turn into a kunit test, to motivate the need for direct
insert-to-swap-cache before running it with the -mm people? It will
otherwise have a high risk of being NAKed, I fear.
*) Trigger immediate writeback of pages handed to the swapcache / shmem,
at least when the shrinker is called from kswapd.
Not sure if that's really valuable.
Not completely sure either. However, in OOM situations where we need to
allocate memory to be able to shrink, that would give the system a
chance to reclaim the pages we shrink before we deplete the kernel
reserves completely. Shmem does this, and also the i915 shrinker in some
situations, but I agree it needs to be verified to be valuable and if
so, in what situations.
*) 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.
Well I think we don't need to split pages at all. The higher order
pages are just allocated for better TLB utilization and could (in
theory) be freed as individual pages as well. It's just that MM
doesn't support that atm.
If we can insert pages directly into the swap-cache, splitting might be
needed, at least if compound pages were allocated to begin with. Looks
like shmem does this as well before inserting into the swap-cache. Could
be a corner case where the system theoretically supports swapping PMD
size pages, but when no PMD size slots are available. (My system behaves
like that, need to investigate why).
Thanks,
Thomas
But I really like the idea of moving more of this logic into the
ttm_pool.
*) 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.
Yeah, that strongly looks like we way to go. Maybe in combination with
being able to swap WC/UC pages directly out.
While swapping them in again an extra copy doesn't hurt us, but for
the other way that really sucks.
Thanks,
Christian.
Thanks,
Thomas
