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

[Ryan Roberts <ryan.roberts@arm.com>]

On 01/03/2024 16:27, 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?

Ahh I just switched to SSD as swap device and now its getting called. I guess
that's the reason. Sorry for the noise.

> 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?
> 
> Thanks,
> Ryan
>