Mina Almasry <almasrymina@xxxxxxxxxx> writes: > On Wed, Nov 23, 2022 at 9:52 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Hi, Johannes, >> >> Johannes Weiner <hannes@xxxxxxxxxxx> writes: >> [...] >> > >> > The fallback to reclaim actually strikes me as wrong. >> > >> > Think of reclaim as 'demoting' the pages to the storage tier. If we >> > have a RAM -> CXL -> storage hierarchy, we should demote from RAM to >> > CXL and from CXL to storage. If we reclaim a page from RAM, it means >> > we 'demote' it directly from RAM to storage, bypassing potentially a >> > huge amount of pages colder than it in CXL. That doesn't seem right. >> > >> > If demotion fails, IMO it shouldn't satisfy the reclaim request by >> > breaking the layering. Rather it should deflect that pressure to the >> > lower layers to make room. This makes sure we maintain an aging >> > pipeline that honors the memory tier hierarchy. >> >> Yes. I think that we should avoid to fall back to reclaim as much as >> possible too. Now, when we allocate memory for demotion >> (alloc_demote_page()), __GFP_KSWAPD_RECLAIM is used. So, we will trigger > > I may be missing something but as far I can tell reclaim is disabled > for allocations from lower tier memory: > https://elixir.bootlin.com/linux/v6.1-rc7/source/mm/vmscan.c#L1583 #define GFP_NOWAIT (__GFP_KSWAPD_RECLAIM) We have GFP_NOWAIT set in gfp. > I think this is maybe a good thing when doing proactive demotion. In > this case we probably don't want to try to reclaim from lower tier > nodes and instead fail the proactive demotion. Do you have some real use cases for this? If so, we can tweak the logic. > However I can see this being desirable when the top tier nodes are > under real memory pressure to deflect that pressure to the lower tier > nodes. Yes. Best Regards, Huang, Ying >> kswapd reclaim on lower tier node to free some memory to avoid fall back >> to reclaim on current (higher tier) node. This may be not good enough, >> for example, the following patch from Hasan may help via waking up >> kswapd earlier. >> >> https://lore.kernel.org/linux-mm/b45b9bf7cd3e21bca61d82dcd1eb692cd32c122c.1637778851.git.hasanalmaruf@xxxxxx/ >> >> Do you know what is the next step plan for this patch? >> >> Should we do even more? >> >> From another point of view, I still think that we can use falling back >> to reclaim as the last resort to avoid OOM in some special situations, >> for example, most pages in the lowest tier node are mlock() or too hot >> to be reclaimed. >> >> > So I'm hesitant to design cgroup controls around the current behavior. > > I sent RFC v2 patch: > https://lore.kernel.org/linux-mm/20221130020328.1009347-1-almasrymina@xxxxxxxxxx/T/#u > > Please take a look when convenient. Thanks! > >> > >> >> Best Regards, >> Huang, Ying >>
