Re: [RFC PATCH V1] mm: Disable demotion from proactive reclaim

[Huang, Ying]

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
>>