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

[Steven Price]

On 24/11/2023 09:01, Ryan Roberts wrote:
> On 24/11/2023 08:55, David Hildenbrand wrote:
>> 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?
> 
> I don;'t think these examples you cite are problematic. Although user space
> thinks about things in 4K pages, the kernel does things in units of folios. So a
> folio is either fully swapped out or not swapped out at all. MTE tags can be
> saved/restored per folio, even if only part of that folio ends up being mapped
> back into user space.
> 
> The problem is that MTE tagging could be active only for a selection of pages
> within the folio; that's where it gets tricky.
> 
>>
>> 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?
> 
> Yes I agree with this approach. I don't know the answer to that question though;
> I'd assume it must be possible. Steven?

Architecturally 'all' pages have MTE metadata (although see Alex's
series[1] which would change this).

The question is: could user space have put any data in the tag storage?
We currently use the PG_mte_tagged page flag to keep track of pages
which have been mapped (to user space) with MTE enabled. If the page has
never been exposed to user space with MTE enabled (since being cleared)
then there's nothing of interest to store.

It would be possible to reverse this scheme - we could drop the page
flag and just look at the actual tag storage. If it's all zeros then
obviously there's no point in storing it. Note that originally we had a
lazy clear of tag storage - i.e. if user space only had mapping without
MTE enabled then the tag storage could contain junk. I believe that's
now changed and the tag storage is always cleared at the same time as
the data storage.

The VMAs (obviously) also carry the information about whether a range is
MTE-enabled (the VM_MTE flag controlled by PROT_MTE in user space), but
there could be many VMAs and they may have different permissions, so
fetching the state from there would be expensive.

Not really relevant here, but the kernel can also use MTE (HW_TAGS
KASAN) - AFAIK there's no way of identifying if the MTE tag storage is
used or not for kernel pages. But this only presents an issue for
hibernation which uses a different mechanism to normal swap.

Steve

[1]
https://lore.kernel.org/r/20231119165721.9849-1-alexandru.elisei%40arm.com