Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole

[Barry Song]

On Tue, Mar 19, 2024 at 5:45 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> >>> I agree phones are not the only platform. But Rome wasn't built in a
> >>> day. I can only get
> >>> started on a hardware which I can easily reach and have enough hardware/test
> >>> resources on it. So we may take the first step which can be applied on
> >>> a real product
> >>> and improve its performance, and step by step, we broaden it and make it
> >>> widely useful to various areas  in which I can't reach :-)
> >>
> >> We must guarantee the normal swap path runs correctly and has no
> >> performance regression when developing SWP_SYNCHRONOUS_IO optimization.
> >> So we have to put some effort on the normal path test anyway.
> >>
> >>> so probably we can have a sysfs "enable" entry with default "n" or
> >>> have a maximum
> >>> swap-in order as Ryan's suggestion [1] at the beginning,
> >>>
> >>> "
> >>> So in the common case, swap-in will pull in the same size of folio as was
> >>> swapped-out. Is that definitely the right policy for all folio sizes? Certainly
> >>> it makes sense for "small" large folios (e.g. up to 64K IMHO). But I'm not sure
> >>> it makes sense for 2M THP; As the size increases the chances of actually needing
> >>> all of the folio reduces so chances are we are wasting IO. There are similar
> >>> arguments for CoW, where we currently copy 1 page per fault - it probably makes
> >>> sense to copy the whole folio up to a certain size.
> >>> "
>
> I thought about this a bit more. No clear conclusions, but hoped this might help
> the discussion around policy:
>
> The decision about the size of the THP is made at first fault, with some help
> from user space and in future we might make decisions to split based on
> munmap/mremap/etc hints. In an ideal world, the fact that we have had to swap
> the THP out at some point in its lifetime should not impact on its size. It's
> just being moved around in the system and the reason for our original decision
> should still hold.

Indeed, this is an ideal framework for smartphones and likely for
widely embedded
Linux systems utilizing zRAM. We set the mTHP size to 64KiB to
leverage CONT-PTE,
given that more than half of the memory on phones may frequently swap out and
swap in (for instance, when opening and switching between apps). The
ideal approach
would involve adhering to the decision made in do_anonymous_page().

>
> So from that PoV, it would be good to swap-in to the same size that was
> swapped-out. But we only kind-of keep that information around, via the swap
> entry contiguity and alignment. With that scheme it is possible that multiple
> virtually adjacent but not physically contiguous folios get swapped-out to
> adjacent swap slot ranges and then they would be swapped-in to a single, larger
> folio. This is not ideal, and I think it would be valuable to try to maintain
> the original folio size information with the swap slot. One way to do this would
> be to store the original order for which the cluster was allocated in the
> cluster. Then we at least know that a given swap slot is either for a folio of
> that order or an order-0 folio (due to cluster exhaustion/scanning). Can we
> steal a bit from swap_map to determine which case it is? Or are there better
> approaches?

In the case of non-SWP_SYNCHRONOUS_IO, users will invariably invoke
swap_readahead()
even when __swap_count(entry) equals 1.  This leads to two scenarios:
swap_vma_readahead
and swap_cluster_readahead.

In swap_vma_readahead, when blk_queue_nonrot, physical contiguity
doesn't appear to be a
critical concern. However, for swap_cluster_readahead, the focus
shifts towards the potential
impact of physical discontiguity.

struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
                                struct vm_fault *vmf)
{
        struct mempolicy *mpol;
        pgoff_t ilx;
        struct folio *folio;

        mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
        folio = swap_use_vma_readahead() ?
                swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf) :
                swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
        mpol_cond_put(mpol);

        if (!folio)
                return NULL;
        return folio_file_page(folio, swp_offset(entry));
}

In Android and embedded systems, SWP_SYNCHRONOUS_IO is consistently utilized,
rendering physical contiguity less of a concern. Moreover, instances where
swap_readahead() is accessed are rare, typically occurring only in scenarios
involving forked but non-CoWed memory.

So I think large folios swap-in will at least need three steps

1. on SWP_SYNCHRONOUS_IO (Android and embedded Linux), this has a very
clear model and has no complex I/O issue.
2. on nonrot block device(bdev_nonrot ==  true), it cares less about
I/O contiguity.
3. on rot block devices which care about  I/O contiguity.

This patchset primarily addresses the systems utilizing
SWP_SYNCHRONOUS_IO(type1),
such as Android and embedded Linux, a straightforward model is established,
with minimal complexity regarding I/O issues.

>
> Next we (I?) have concerns about wasting IO by swapping-in folios that are too
> large (e.g. 2M). I'm not sure if this is a real problem or not - intuitively I'd
> say yes but I have no data. But on the other hand, memory is aged and
> swapped-out per-folio, so why shouldn't it be swapped-in per folio? If the
> original allocation size policy is good (it currently isn't) then a folio should
> be sized to cover temporally close memory and if we need to access some of it,
> chances are we need all of it.
>
> If we think the IO concern is legitimate then we could define a threshold size
> (sysfs?) for when we start swapping-in the folio in chunks. And how big should
> those chunks be - one page, or the threshold size itself? Probably the latter?
> And perhaps that threshold could also be used by zRAM to decide its upper limit
> for compression chunk.


Agreed. What about introducing a parameter like
/sys/kernel/mm/transparent_hugepage/max_swapin_order
giving users the opportunity to fine-tune it according to their needs. For type1
users specifically, setting it to any value above 4 would be
beneficial. If there's
still a lack of tuning for desktop and server environments (type 2 and type 3),
the default value could be set to 0.

>
> Perhaps we can learn from khugepaged here? I think it has programmable
> thresholds for how many swapped-out pages can be swapped-in to aid collapse to a
> THP? I guess that exists for the same concerns about increased IO pressure?
>
>
> If we think we will ever be swapping-in folios in chunks less than their
> original size, then we need a separate mechanism to re-foliate them. We have
> discussed a khugepaged-like approach for doing this asynchronously in the
> background. I know that scares the Android folks, but David has suggested that
> this could well be very cheap compared with khugepaged, because it would be
> entirely limited to a single pgtable, so we only need the PTL. If we need this
> mechanism anyway, perhaps we should develop it and see how it performs if
> swap-in remains order-0? Although I guess that would imply not being able to
> benefit from compressing THPs for the zRAM case.

The effectiveness of collapse operation relies on the stability of
forming large folios
to ensure optimal performance. In embedded systems, where more than half of the
memory may be allocated to zRAM, folios might undergo swapping out before
collapsing or immediately after the collapse operation. It seems a
TAO-like optimization
to decrease fallback and latency is more effective.

>
> I see all this as orthogonal to synchronous vs asynchronous swap devices. I
> think the latter just implies that you might want to do some readahead to try to
> cover up the latency? If swap is moving towards being folio-orientated, then
> readahead also surely needs to be folio-orientated, but I think that should be
> the only major difference.
>
> Anyway, just some thoughts!

Thank you very much for your valuable and insightful deliberations.

>
> Thanks,
> Ryan
>

Thanks
Barry