Re: [PATCH v2 00/17] mm: fixes for device-exclusive entries (hmm)

[Alistair Popple <apopple@nvidia.com>]

On Thu, Feb 13, 2025 at 12:15:58PM +0100, David Hildenbrand wrote:
> On 13.02.25 12:03, Alistair Popple wrote:
> > On Mon, Feb 10, 2025 at 08:37:42PM +0100, David Hildenbrand wrote:
> > > Against mm-hotfixes-stable for now.
> > > 
> > > Discussing the PageTail() call in make_device_exclusive_range() with
> > > Willy, I recently discovered [1] that device-exclusive handling does
> > > not properly work with THP, making the hmm-tests selftests fail if THPs
> > > are enabled on the system.
> > > 
> > > Looking into more details, I found that hugetlb is not properly fenced,
> > > and I realized that something that was bugging me for longer -- how
> > > device-exclusive entries interact with mapcounts -- completely breaks
> > > migration/swapout/split/hwpoison handling of these folios while they have
> > > device-exclusive PTEs.
> > > 
> > > The program below can be used to allocate 1 GiB worth of pages and
> > > making them device-exclusive on a kernel with CONFIG_TEST_HMM.
> > > 
> > > Once they are device-exclusive, these folios cannot get swapped out
> > > (proc$pid/smaps_rollup will always indicate 1 GiB RSS no matter how
> > > much one forces memory reclaim), and when having a memory block onlined
> > > to ZONE_MOVABLE, trying to offline it will loop forever and complain about
> > > failed migration of a page that should be movable.
> > > 
> > > # echo offline > /sys/devices/system/memory/memory136/state
> > > # echo online_movable > /sys/devices/system/memory/memory136/state
> > > # ./hmm-swap &
> > > ... wait until everything is device-exclusive
> > > # echo offline > /sys/devices/system/memory/memory136/state
> > > [  285.193431][T14882] page: refcount:2 mapcount:0 mapping:0000000000000000
> > >    index:0x7f20671f7 pfn:0x442b6a
> > > [  285.196618][T14882] memcg:ffff888179298000
> > > [  285.198085][T14882] anon flags: 0x5fff0000002091c(referenced|uptodate|
> > >    dirty|active|owner_2|swapbacked|node=1|zone=3|lastcpupid=0x7ff)
> > > [  285.201734][T14882] raw: ...
> > > [  285.204464][T14882] raw: ...
> > > [  285.207196][T14882] page dumped because: migration failure
> > > [  285.209072][T14882] page_owner tracks the page as allocated
> > > [  285.210915][T14882] page last allocated via order 0, migratetype
> > >    Movable, gfp_mask 0x140dca(GFP_HIGHUSER_MOVABLE|__GFP_COMP|__GFP_ZERO),
> > >    id 14926, tgid 14926 (hmm-swap), ts 254506295376, free_ts 227402023774
> > > [  285.216765][T14882]  post_alloc_hook+0x197/0x1b0
> > > [  285.218874][T14882]  get_page_from_freelist+0x76e/0x3280
> > > [  285.220864][T14882]  __alloc_frozen_pages_noprof+0x38e/0x2740
> > > [  285.223302][T14882]  alloc_pages_mpol+0x1fc/0x540
> > > [  285.225130][T14882]  folio_alloc_mpol_noprof+0x36/0x340
> > > [  285.227222][T14882]  vma_alloc_folio_noprof+0xee/0x1a0
> > > [  285.229074][T14882]  __handle_mm_fault+0x2b38/0x56a0
> > > [  285.230822][T14882]  handle_mm_fault+0x368/0x9f0
> > > ...
> > > 
> > > This series fixes all issues I found so far. There is no easy way to fix
> > > without a bigger rework/cleanup. I have a bunch of cleanups on top (some
> > > previous sent, some the result of the discussion in v1) that I will send
> > > out separately once this landed and I get to it.
> > > I wish we could just use some special present PROT_NONE PTEs instead of

Yeah, that was my initial instinct when I first investigated this. As you point
out a lack of spare PTE bits made it hard/impossible. Of course I'm about to
give you all one back, maybe I should keep it :) I'm only kidding though - I'm
sure there's more interesting things to spend it on.

> > 
> > First off David thanks for finding and fixing these issues. If you have further
> > clean-ups in mind that you need help with please let me know as I'd be happy
> > to help.
> 
> Sure! I have some cleanups TBD as result of the previous discussion, but
> nothing bigger so far.
> 
> (removing the folio lock could be considered bigger, if we want to go down
> that path)
> 
> > 
> > > these (non-present, non-none) fake-swap entries; but that just results in
> > > the same problem we keep having (lack of spare PTE bits), and staring at
> > > other similar fake-swap entries, that ship has sailed.
> > > 
> > > With this series, make_device_exclusive() doesn't actually belong into
> > > mm/rmap.c anymore, but I'll leave moving that for another day.
> > > 
> > > I only tested this series with the hmm-tests selftests due to lack of HW,
> > > so I'd appreciate some testing, especially if the interaction between
> > > two GPUs wanting a device-exclusive entry works as expected.
> > 
> > I'm still reviewing the series but so far testing on my single GPU system
> > appears to be working as expected. I will try and fire up a dual GPU system
> > tomorrow and test it there as well.
> 
> Great, thanks a bunch for testing!
> 
> Out of interest: does the nvidia driver make use of this interface as well,
> and are you testing with that or with the nouveau driver? I saw some reports
> that nvidia at least checks for it [1] when building the module:

Both. I have tested Nouveau with the Mesa OpenCL stack and a simple stress test
that just thrashes atomic accesses between CPU and GPU and a similar test for
the nvidia driver.

In practice the nvidia driver probably doesn't use this that often as it
more aggressively migrates data but it does use this as a fallback. Also it's
possible for users to force residency on the CPU in which case this is used,
which is what the test does.

Anyway I have just finished testing on a multi-GPU setup so please feel free to
add for the series:

Tested-by: Alistair Popple <apopple@nvidia.com>

> 
> 	CONFTEST: make_device_exclusive_range
> 
> [1] https://www.googlecloudcommunity.com/gc/AI-ML/Can-t-Install-Nvidia-Drivers-on-6-1-0-18-Kernel/m-p/722596
> 
> -- 
> Cheers,
> 
> David / dhildenb
>