Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

[David Hildenbrand]

On 28.02.24 15:57, Ryan Roberts wrote:
> On 28/02/2024 12:12, David Hildenbrand wrote:
> > > > How relevant is it? Relevant enough that someone decided to put that
> > > > optimization in? I don't know :)
> > >
> > > I'll have one last go at convincing you: Huang Ying (original author) commented
> > > "I believe this should be OK.  Better to compare the performance too." at [1].
> > > That implies to me that perhaps the optimization wasn't in response to a
> > > specific problem after all. Do you have any thoughts, Huang?
> >
> > Might make sense to include that in the patch description!
> >
> > > OK so if we really do need to keep this optimization, here are some ideas:
> > >
> > > Fundamentally, we would like to be able to figure out the size of the swap slot
> > > from the swap entry. Today swap supports 2 sizes; PAGE_SIZE and PMD_SIZE. For
> > > PMD_SIZE, it always uses a full cluster, so can easily add a flag to the cluster
> > > to mark it as PMD_SIZE.
> > >
> > > Going forwards, we want to support all sizes (power-of-2). Most of the time, a
> > > cluster will contain only one size of THPs, but this is not the case when a THP
> > > in the swapcache gets split or when an order-0 slot gets stolen. We expect these
> > > cases to be rare.
> > >
> > > 1) Keep the size of the smallest swap entry in the cluster header. Most of the
> > > time it will be the full size of the swap entry, but sometimes it will cover
> > > only a portion. In the latter case you may see a false negative for
> > > swap_page_trans_huge_swapped() meaning we take the slow path, but that is rare.
> > > There is one wrinkle: currently the HUGE flag is cleared in put_swap_folio(). We
> > > wouldn't want to do the equivalent in the new scheme (i.e. set the whole cluster
> > > to order-0). I think that is safe, but haven't completely convinced myself yet.
> > >
> > > 2) allocate 4 bits per (small) swap slot to hold the order. This will give
> > > precise information and is conceptually simpler to understand, but will cost
> > > more memory (half as much as the initial swap_map[] again).
> > >
> > > I still prefer to avoid this at all if we can (and would like to hear Huang's
> > > thoughts). But if its a choice between 1 and 2, I prefer 1 - I'll do some
> > > prototyping.
> >
> > Taking a step back: what about we simply batch unmapping of swap entries?
> >
> > That is, if we're unmapping a PTE range, we'll collect swap entries (under PT
> > lock) that reference consecutive swap offsets in the same swap file.
>
> Yes in principle, but there are 4 places where free_swap_and_cache() is called,
> and only 2 of those are really amenable to batching (zap_pte_range() and
> madvise_free_pte_range()). So the other two users will still take the "slow"
> path. Maybe those 2 callsites are the only ones that really matter? I can
> certainly have a stab at this approach.

We can ignore the s390x one. That s390x code should only apply to KVM 
guest memory where ordinary THP are not even supported. (and nobody uses 
mTHP there yet).

Long story short: the VM can hint that some memory pages are now unused 
and the hypervisor can reclaim them. That's what that callback does (zap 
guest-provided guest memory). No need to worry about any batching for now.

Then, there is the shmem one in shmem_free_swap(). I really don't know 
how shmem handles THP+swapout.

But looking at shmem_writepage(), we split any large folios before 
moving them to the swapcache, so likely we don't care at all, because 
THP don't apply.

> > There, we can then first decrement all the swap counts, and then try minimizing
> > how often we actually have to try reclaiming swap space (lookup folio, see it's
> > a large folio that we cannot reclaim or could reclaim, ...).
> >
> > Might need some fine-tuning in swap code to "advance" to the next entry to try
> > freeing up, but we certainly can do better than what we would do right now.
>
> I'm not sure I've understood this. Isn't advancing just a matter of:
>
> entry = swp_entry(swp_type(entry), swp_offset(entry) + 1);

I was talking about the advancing swapslot processing after decrementing 
the swapcounts.

Assume you decremented 512 swapcounts and some of them went to 0. AFAIU, 
you'd have to start with the first swapslot that has now a swapcount=0 
one and try to reclaim swap.

Assume you get a small folio, then you'll have to proceed with the next 
swap slot and try to reclaim swap.

Assume you get a large folio, then you can skip more swapslots 
(depending on offset into the folio etc).

If you get what I mean. :)

--
Cheers,

David / dhildenb