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

[Barry Song]

On Fri, Nov 24, 2023 at 10:55 PM Steven Price <steven.price@xxxxxxx> wrote:
>
> 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.

I am not so optimistic :-)

but zap_pte_range() due to DONTNEED on a part of swapped-out folio can
free a part of swap
entries? thus, free a part of MTE tags in a folio?
after process's large folios are swapped out, all PTEs in a large
folio become swap
entries, but DONTNEED on a part of this area will only set a part of
swap entries to
PTE_NONE, thus decrease the swapcount of this part?

zap_pte_range
    ->
          entry = pte_to_swp_entry
                  -> free_swap_and_cache(entry)
                      -> mte tags invalidate

> >
> > 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

Thanks
Barry