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

[David Hildenbrand <david@redhat.com>]

On 04.03.24 17:03, 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.
>>
>> 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.
>>
> 
> Hi,
> 
> I'm struggling to convince myself that free_swap_and_cache() can't race with
> with swapoff(). Can anyone explain that this is safe?
> 
> I *think* they are both serialized by the PTL, since all callers of
> free_swap_and_cache() (except shmem) have the PTL, and swapoff() calls
> try_to_unuse() early on, which takes the PTL as it iterates over every vma in
> every mm. It looks like shmem is handled specially by a call to shmem_unuse(),
> but I can't see the exact serialization mechanism.

As get_swap_device() documents:

"if there aren't some other ways to prevent swapoff, such as the folio 
in swap cache is locked, page table lock is held, etc., the swap entry 
may become invalid because of swapoff"

PTL it is, in theory. But I'm afraid that's half the story.

> 
> I've implemented a batching function, as David suggested above, but I'm trying
> to convince myself that it is safe for it to access si->swap_map[] without a
> lock (i.e. that swapoff() can't concurrently free it). But I think
> free_swap_and_cache() as it already exists depends on being able to access the
> si without an explicit lock, so I'm assuming the same mechanism will protect my
> new changes. But I want to be sure I understand the mechanism...

Very valid concern.

> 
> 
> This is the existing free_swap_and_cache(). I think _swap_info_get() would break
> if this could race with swapoff(), and __swap_entry_free() looks up the cluster
> from an array, which would also be freed by swapoff if racing:
> 
> 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 dropped to 0 and

> 		if (count == SWAP_HAS_CACHE)


count is now SWAP_HAS_CACHE, there is in fact no swap entry anymore. We 
removed it. That one would have to be reclaimed asynchronously.

The existing code we would call swap_page_trans_huge_swapped() with the 
SI it obtained via _swap_info_get().

I also don't see what should be left protecting the SI. It's not locked 
anymore, the swapcounts are at 0. We don't hold the folio lock.

try_to_unuse() will stop as soon as si->inuse_pages is at 0. Hm ...

Would performing the overall operation under lock_cluster_or_swap_info 
help? Not so sure :(

-- 
Cheers,

David / dhildenb