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

From: David Hildenbrand
Date: Fri Nov 24 2023 - 03:55:23 EST

On 24.11.23 02:35, Barry Song wrote:
On Mon, Nov 20, 2023 at 11:57 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:

On 20/11/2023 09:11, David Hildenbrand wrote:
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.

We already have a page flag that we use to mark the page as having had its mte
state associated; PG_mte_tagged. This is currently per-page (and IIUC, Matthew
has been working to remove as many per-page flags as possible). Couldn't we just
make arch_swap_restore() take a folio, restore the tags for *all* the pages and
repurpose that flag to be per-folio (so head page only)? It looks like the the
mte code already manages all the serialization requirements too. Then
arch_swap_restore() can just exit early if it sees the flag is already set on
the folio.

One (probably nonsense) concern that just sprung to mind about having MTE work
with large folios in general; is it possible that user space could cause a large
anon folio to be allocated (THP), then later mark *part* of it to be tagged with
MTE? In this case you would need to apply tags to part of the folio only.
Although I have a vague recollection that any MTE areas have to be marked at
mmap time and therefore this type of thing is impossible?

right, we might need to consider only a part of folio needs to be
mapped and restored MTE tags.
do_swap_page() can have a chance to hit a large folio but it only
needs to fault-in a page.

A case can be quite simple as below,

1. anon folio shared by process A and B
2. add_to_swap() as a large folio;
3. try to unmap A and B;
4. after A is unmapped(ptes become swap entries), we do a
MADV_DONTNEED on a part of the folio. this can
happen very easily as userspace is still working in 4KB level;
userspace heap management can free an
basepage area by MADV_DONTNEED;
madvise(address, MADV_DONTNEED, 4KB);
5. A refault on address + 8KB, we will hit large folio in
do_swap_page() but we will only need to map
one basepage, we will never need this DONTNEEDed in process A.

another more complicated case can be mprotect and munmap a part of
large folios. since userspace
has no idea of large folios in their mind, they can do all strange
things. are we sure in all cases,
large folios have been splitted into small folios?

To handle that, we'd have to identify

a) if a subpage has an mte tag to save during swapout
b) if a subpage has an mte tag to restore during swapin

I suspect b) can be had from whatever datastructure we're using to actually save the tags?

For a), is there some way to have that information from the HW?

--
Cheers,

David / dhildenb