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

[David Hildenbrand]

On 01.03.24 17:27, Ryan Roberts wrote:
> On 28/02/2024 15:12, David Hildenbrand wrote:
> > 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. :)
>
> I've implemented the batching as David suggested, and I'm pretty confident it's
> correct. The only problem is that during testing I can't provoke the code to
> take the path. I've been pouring through the code but struggling to figure out
> under what situation you would expect the swap entry passed to
> free_swap_and_cache() to still have a cached folio? Does anyone have any idea?
>
> This is the original (unbatched) function, after my change, which caused David's
> concern that we would end up calling __try_to_reclaim_swap() far too much:
>
> int free_swap_and_cache(swp_entry_t entry)
> {
> 	struct swap_info_struct *p;
> 	unsigned char count;
>
> 	if (non_swap_entry(entry))
> 		return 1;
>
> 	p = _swap_info_get(entry);
> 	if (p) {
> 		count = __swap_entry_free(p, entry);
> 		if (count == SWAP_HAS_CACHE)
> 			__try_to_reclaim_swap(p, swp_offset(entry),
> 					      TTRS_UNMAPPED | TTRS_FULL);
> 	}
> 	return p != NULL;
> }
>
> The trouble is, whenever its called, count is always 0, so
> __try_to_reclaim_swap() never gets called.
>
> My test case is allocating 1G anon memory, then doing madvise(MADV_PAGEOUT) over
> it. Then doing either a munmap() or madvise(MADV_FREE), both of which cause this
> function to be called for every PTE, but count is always 0 after
> __swap_entry_free() so __try_to_reclaim_swap() is never called. I've tried for
> order-0 as well as PTE- and PMD-mapped 2M THP.
>
> I'm guessing the swapcache was already reclaimed as part of MADV_PAGEOUT? I'm
> using a block ram device as my backing store - I think this does synchronous IO
> so perhaps if I have a real block device with async IO I might have more luck?
> Just a guess...
>
> Or perhaps this code path is a corner case? In which case, perhaps its not worth
> adding the batching optimization after all?

I had to disable zswap in the past and was able to trigger this reliably 
with an ordinary swap backend (e.g., proper disk).

Whenever you involve swap-to-ram, you might just get it reclaimed 
immediately.

--
Cheers,

David / dhildenb