Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios

From: David Hildenbrand
Date: Mon Nov 20 2023 - 04:12:24 EST

On 17.11.23 19:41, Barry Song wrote:
On Fri, Nov 17, 2023 at 7:28 PM David Hildenbrand <david@xxxxxxxxxx> wrote:

On 17.11.23 01:15, Barry Song wrote:
On Fri, Nov 17, 2023 at 7:47 AM Barry Song <21cnbao@xxxxxxxxx> wrote:

On Thu, Nov 16, 2023 at 5:36 PM David Hildenbrand <david@xxxxxxxxxx> wrote:

On 15.11.23 21:49, Barry Song wrote:
On Wed, Nov 15, 2023 at 11:16 PM David Hildenbrand <david@xxxxxxxxxx> wrote:

On 14.11.23 02:43, Barry Song wrote:
This patch makes MTE tags saving and restoring support large folios,
then we don't need to split them into base pages for swapping out
on ARM64 SoCs with MTE.

arch_prepare_to_swap() should take folio rather than page as parameter
because we support THP swap-out as a whole.

Meanwhile, arch_swap_restore() should use page parameter rather than
folio as swap-in always works at the granularity of base pages right
now.

... but then we always have order-0 folios and can pass a folio, or what
am I missing?

Hi David,
you missed the discussion here:

https://lore.kernel.org/lkml/CAGsJ_4yXjex8txgEGt7+WMKp4uDQTn-fR06ijv4Ac68MkhjMDw@xxxxxxxxxxxxxx/
https://lore.kernel.org/lkml/CAGsJ_4xmBAcApyK8NgVQeX_Znp5e8D4fbbhGguOkNzmh1Veocg@xxxxxxxxxxxxxx/

Okay, so you want to handle the refault-from-swapcache case where you get a
large folio.

I was mislead by your "folio as swap-in always works at the granularity of
base pages right now" comment.

What you actually wanted to say is "While we always swap in small folios, we
might refault large folios from the swapcache, and we only want to restore
the tags for the page of the large folio we are faulting on."

But, I do if we can't simply restore the tags for the whole thing at once
at make the interface page-free?

Let me elaborate:

IIRC, if we have a large folio in the swapcache, the swap entries/offset are
contiguous. If you know you are faulting on page[1] of the folio with a
given swap offset, you can calculate the swap offset for page[0] simply by
subtracting from the offset.

See page_swap_entry() on how we perform this calculation.


So you can simply pass the large folio and the swap entry corresponding
to the first page of the large folio, and restore all tags at once.

So the interface would be

arch_prepare_to_swap(struct folio *folio);
void arch_swap_restore(struct page *folio, swp_entry_t start_entry);

I'm sorry if that was also already discussed.

This has been discussed. Steven, Ryan and I all don't think this is a good
option. in case we have a large folio with 16 basepages, as do_swap_page
can only map one base page for each page fault, that means we have
to restore 16(tags we restore in each page fault) * 16(the times of page faults)
for this large folio.

and still the worst thing is the page fault in the Nth PTE of large folio
might free swap entry as that swap has been in.
do_swap_page()
{
/*
* Remove the swap entry and conditionally try to free up the swapcache.
* We're already holding a reference on the page but haven't mapped it
* yet.
*/
swap_free(entry);
}

So in the page faults other than N, I mean 0~N-1 and N+1 to 15, you might access
a freed tag.

And David, one more information is that to keep the parameter of
arch_swap_restore() unchanged as folio,
i actually tried an ugly approach in rfc v2:

+void arch_swap_restore(swp_entry_t entry, struct folio *folio)
+{
+ if (system_supports_mte()) {
+ /*
+ * We don't support large folios swap in as whole yet, but
+ * we can hit a large folio which is still in swapcache
+ * after those related processes' PTEs have been unmapped
+ * but before the swapcache folio is dropped, in this case,
+ * we need to find the exact page which "entry" is mapping
+ * to. If we are not hitting swapcache, this folio won't be
+ * large
+ */
+ struct page *page = folio_file_page(folio, swp_offset(entry));
+ mte_restore_tags(entry, page);
+ }
+}

And obviously everybody in the discussion hated it :-)


I can relate :D

i feel the only way to keep API unchanged using folio is that we
support restoring PTEs
all together for the whole large folio and we support the swap-in of
large folios. This is
in my list to do, I will send a patchset based on Ryan's large anon
folios series after a
while. till that is really done, it seems using page rather than folio
is a better choice.

I think just restoring all tags and remembering for a large folio that
they have been restored might be the low hanging fruit. But as always,
devil is in the detail :)

Hi David,
thanks for all your suggestions though my feeling is this is too complex and
is not worth it for at least three reasons.

Fair enough.


1. In multi-thread and particularly multi-processes, we need some locks to
protect and help know if one process is the first one to restore tags and if
someone else is restoring tags when one process wants to restore. there
is not this kind of fine-grained lock at all.

We surely always hold the folio lock on swapin/swapout, no? So when these functions are called.

So that might just work already -- unless I am missing something important.


2. folios are not always large, in many cases, they have just one base page
and there is no tail to remember. and it seems pretty ugly if we turn out have
to use different ways to remember restoring state for small folios and
large folios.

if (folio_test_large(folio)) {

} else {

}

Easy ;)

Seriously, it's not that complicated and/or ugly.


3. there is nothing wrong to use page to restore tags since right now swap-in
is page. restoring tags and swapping-in become harmonious with each other
after this patch. I would argue what is really wrong is the current mainline.

If eventually we are able to do_swap_page() for the whole large folio, we
move to folios for MTE tags as well. These two behaviours make a new
harmonious picture again.


Just making both functions consume folios is much cleaner and also more future proof.

Consuming now a page where we used to consume a folio is a step backwards.

--
Cheers,

David / dhildenb