Re: [PATCH v4 13/13] mm/gup: Handle hugetlb in the generic follow_page_mask code

[Ryan Roberts <ryan.roberts@arm.com>]

On 02/04/2024 17:20, Peter Xu wrote:
> On Tue, Apr 02, 2024 at 05:26:28PM +0200, David Hildenbrand wrote:
>> On 02.04.24 16:48, Ryan Roberts wrote:
>>> Hi Peter,
> 
> Hey, Ryan,
> 
> Thanks for the report!
> 
>>>
>>> On 27/03/2024 15:23, peterx@redhat.com wrote:
>>>> From: Peter Xu <peterx@redhat.com>
>>>>
>>>> Now follow_page() is ready to handle hugetlb pages in whatever form, and
>>>> over all architectures.  Switch to the generic code path.
>>>>
>>>> Time to retire hugetlb_follow_page_mask(), following the previous
>>>> retirement of follow_hugetlb_page() in 4849807114b8.
>>>>
>>>> There may be a slight difference of how the loops run when processing slow
>>>> GUP over a large hugetlb range on cont_pte/cont_pmd supported archs: each
>>>> loop of __get_user_pages() will resolve one pgtable entry with the patch
>>>> applied, rather than relying on the size of hugetlb hstate, the latter may
>>>> cover multiple entries in one loop.
>>>>
>>>> A quick performance test on an aarch64 VM on M1 chip shows 15% degrade over
>>>> a tight loop of slow gup after the path switched.  That shouldn't be a
>>>> problem because slow-gup should not be a hot path for GUP in general: when
>>>> page is commonly present, fast-gup will already succeed, while when the
>>>> page is indeed missing and require a follow up page fault, the slow gup
>>>> degrade will probably buried in the fault paths anyway.  It also explains
>>>> why slow gup for THP used to be very slow before 57edfcfd3419 ("mm/gup:
>>>> accelerate thp gup even for "pages != NULL"") lands, the latter not part of
>>>> a performance analysis but a side benefit.  If the performance will be a
>>>> concern, we can consider handle CONT_PTE in follow_page().
>>>>
>>>> Before that is justified to be necessary, keep everything clean and simple.
>>>>
>>>> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
>>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>>
>>> Afraid I'm seeing an oops when running gup_longterm test on arm64 with current mm-unstable. Git bisect blames this patch. The oops reproduces for me every time on 2 different machines:
>>>
>>>
>>> [    9.340416] kernel BUG at mm/gup.c:778!
>>> [    9.340746] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
>>> [    9.341199] Modules linked in:
>>> [    9.341481] CPU: 1 PID: 1159 Comm: gup_longterm Not tainted 6.9.0-rc2-00210-g910ff1a347e4 #11
>>> [    9.342232] Hardware name: linux,dummy-virt (DT)
>>> [    9.342647] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>> [    9.343195] pc : follow_page_mask+0x4d4/0x880
>>> [    9.343580] lr : follow_page_mask+0x4d4/0x880
>>> [    9.344018] sp : ffff8000898b3aa0
>>> [    9.344345] x29: ffff8000898b3aa0 x28: fffffdffc53973e8 x27: 00003c0005d08000
>>> [    9.345028] x26: ffff00014e5cfd08 x25: ffffd3513a40c000 x24: fffffdffc5d08000
>>> [    9.345682] x23: ffffc1ffc0000000 x22: 0000000000080101 x21: ffff8000898b3ba8
>>> [    9.346337] x20: 0000fffff4200000 x19: ffff00014e52d508 x18: 0000000000000010
>>> [    9.347005] x17: 5f656e6f7a5f7369 x16: 2120262620296567 x15: 6170286461654865
>>> [    9.347713] x14: 6761502128454741 x13: 2929656761702865 x12: 6761705f65636976
>>> [    9.348371] x11: 65645f656e6f7a5f x10: ffffd3513b31d6e0 x9 : ffffd3513852f090
>>> [    9.349062] x8 : 00000000ffffefff x7 : ffffd3513b31d6e0 x6 : 0000000000000000
>>> [    9.349753] x5 : ffff00017ff98cc8 x4 : 0000000000000fff x3 : 0000000000000000
>>> [    9.350397] x2 : 0000000000000000 x1 : ffff000190e8b480 x0 : 0000000000000052
>>> [    9.351097] Call trace:
>>> [    9.351312]  follow_page_mask+0x4d4/0x880
>>> [    9.351700]  __get_user_pages+0xf4/0x3e8
>>> [    9.352089]  __gup_longterm_locked+0x204/0xa70
>>> [    9.352516]  pin_user_pages+0x88/0xc0
>>> [    9.352873]  gup_test_ioctl+0x860/0xc40
>>> [    9.353249]  __arm64_sys_ioctl+0xb0/0x100
>>> [    9.353648]  invoke_syscall+0x50/0x128
>>> [    9.354022]  el0_svc_common.constprop.0+0x48/0xf8
>>> [    9.354488]  do_el0_svc+0x28/0x40
>>> [    9.354822]  el0_svc+0x34/0xe0
>>> [    9.355128]  el0t_64_sync_handler+0x13c/0x158
>>> [    9.355489]  el0t_64_sync+0x190/0x198
>>> [    9.355793] Code: aa1803e0 d000d8e1 91220021 97fff560 (d4210000)
>>> [    9.356280] ---[ end trace 0000000000000000 ]---
>>> [    9.356651] note: gup_longterm[1159] exited with irqs disabled
>>> [    9.357141] note: gup_longterm[1159] exited with preempt_count 2
>>> [    9.358033] ------------[ cut here ]------------
>>> [    9.358800] WARNING: CPU: 1 PID: 0 at kernel/context_tracking.c:128 ct_kernel_exit.constprop.0+0x108/0x120
>>> [    9.360157] Modules linked in:
>>> [    9.360541] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G      D            6.9.0-rc2-00210-g910ff1a347e4 #11
>>> [    9.361626] Hardware name: linux,dummy-virt (DT)
>>> [    9.362087] pstate: 204003c5 (nzCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>> [    9.362758] pc : ct_kernel_exit.constprop.0+0x108/0x120
>>> [    9.363306] lr : ct_idle_enter+0x10/0x20
>>> [    9.363845] sp : ffff8000801abdc0
>>> [    9.364222] x29: ffff8000801abdc0 x28: 0000000000000000 x27: 0000000000000000
>>> [    9.364961] x26: 0000000000000000 x25: ffff00014149d780 x24: 0000000000000000
>>> [    9.365557] x23: 0000000000000000 x22: ffffd3513b299d48 x21: ffffd3513a785730
>>> [    9.366239] x20: ffffd3513b299c28 x19: ffff00017ffa7da0 x18: 0000fffff5ffffff
>>> [    9.366869] x17: 0000000000000000 x16: 1fffe0002a21a8c1 x15: 0000000000000000
>>> [    9.367524] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000002
>>> [    9.368207] x11: 0000000000000001 x10: 0000000000000ad0 x9 : ffffd35138589230
>>> [    9.369123] x8 : ffff00014149e2b0 x7 : 0000000000000000 x6 : 000000000f8c0fb2
>>> [    9.370403] x5 : 4000000000000002 x4 : ffff2cb045825000 x3 : ffff8000801abdc0
>>> [    9.371170] x2 : ffffd3513a782da0 x1 : 4000000000000000 x0 : ffffd3513a782da0
>>> [    9.372279] Call trace:
>>> [    9.372519]  ct_kernel_exit.constprop.0+0x108/0x120
>>> [    9.373216]  ct_idle_enter+0x10/0x20
>>> [    9.373562]  default_idle_call+0x3c/0x160
>>> [    9.374055]  do_idle+0x21c/0x280
>>> [    9.374394]  cpu_startup_entry+0x3c/0x50
>>> [    9.374797]  secondary_start_kernel+0x140/0x168
>>> [    9.375220]  __secondary_switched+0xb8/0xc0
>>> [    9.375875] ---[ end trace 0000000000000000 ]---
>>>
>>>
>>> The oops trigger is at mm/gup.c:778:
>>> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
>>>
>>>
>>> This is the output of gup_longterm (last output is just before oops):
>>>
>>> # [INFO] detected hugetlb page size: 2048 KiB
>>> # [INFO] detected hugetlb page size: 32768 KiB
>>> # [INFO] detected hugetlb page size: 64 KiB
>>> # [INFO] detected hugetlb page size: 1048576 KiB
>>> TAP version 13
>>> 1..70
>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd
>>> ok 1 Should have worked
>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with tmpfile
>>> ok 2 Should have failed
>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile
>>> ok 3 Should have failed
>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
>>> ok 4 Should have worked
>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (32768 kB)
>>>
>>>
>>> So 2M passed ok, and its failing for 32M, which is cont-pmd. I'm guessing you're trying to iterate 2M into a cont-pmd folio and ending up with an unexpected tail page?
>>
>> I assume we find the expected tail page, it's just that the check
>>
>> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
>>
>> Doesn't make sense with hugetlb folios. We might have a tail page mapped in
>> a cont-pmd entry. As soon as we call follow_huge_pmd() on "not the first
>> cont-pmd entry", we trigger this check.
>>
>> Likely this sanity check must also allow for hugetlb folios. Or we should
>> just remove it completely.
> 
> Right, IMHO it'll be easier we remove it, actually I see there's one more
> at the end, so I think we need to remove both.
> 
>>
>> In the past, we wanted to make sure that we never get tail pages of THP from
>> PMD entries, because something would currently be broken (we don't support
>> THP > PMD).
> 
> There's probably one more thing we need to do, on allowing
> PageAnonExclusive() to work with hugetlb tails. Even if we remove the
> warnings and if I read the code right, we can BUG_ON again on checking tail
> pages over anon-exclusive for PageHuge.
> 
> So I assume to fix it completely, we may need two changes: Patch 1 to
> prepare PageAnonExclusive() to work on hugetlb tails, then patch 2 to be
> squashed into the patch "mm/gup: handle huge pmd for follow_pmd_mask()".
> Note: not this patch to fixup, as this patch only does the "switchover" to
> the new path, the culprit should be the other patch..

I'll leave you to do the testing on this, if that's ok.

Just to make my config explicit, I have this kernel command line, which reserves
hugetlbs of all sizes for the tests:

"transparent_hugepage=madvise earlycon root=/dev/vda2 secretmem.enable
hugepagesz=1G hugepages=0:2,1:2 hugepagesz=32M hugepages=0:2,1:2
default_hugepagesz=2M hugepages=0:64,1:64 hugepagesz=64K hugepages=0:2,1:2"

Thanks,
Ryan

> 
> I have them attached below first, before I'll also go and see whether I can
> run some arm tests later today or tomorrow.  David, any comments from
> anon-exclusive side?
> 
> Thanks,
> 
> ===8<===
> 
> From 26f0670acea948945222c97a9cab58428782ca69 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@redhat.com>
> Date: Tue, 2 Apr 2024 11:52:28 -0400
> Subject: [PATCH 1/2] mm: Allow anon exclusive check over hugetlb tail pages
> 
> PageAnonExclusive() used to forbid tail pages for hugetlbfs, as that used
> to be called mostly in hugetlb specific paths and the head page was
> guaranteed.
> 
> As we move forward towards merging hugetlb paths into generic mm, we may
> start to pass in tail hugetlb pages (when with cont-pte/cont-pmd huge
> pages) for such check.  Allow it to properly fetch the head, in which case
> the anon-exclusiveness of the head will always represents the tail page.
> 
> There's already a sign of it when we look at the fast-gup which already
> contain the hugetlb processing altogether: we used to have a specific
> commit 5805192c7b72 ("mm/gup: handle cont-PTE hugetlb pages correctly in
> gup_must_unshare() via GUP-fast") covering that area.  Now with this more
> generic change, that can also go away.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/linux/page-flags.h |  8 +++++++-
>  mm/internal.h              | 10 ----------
>  2 files changed, 7 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 888353c209c0..225357f48a79 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -1095,7 +1095,13 @@ PAGEFLAG(Isolated, isolated, PF_ANY);
>  static __always_inline int PageAnonExclusive(const struct page *page)
>  {
>  	VM_BUG_ON_PGFLAGS(!PageAnon(page), page);
> -	VM_BUG_ON_PGFLAGS(PageHuge(page) && !PageHead(page), page);
> +	/*
> +	 * Allow the anon-exclusive check to work on hugetlb tail pages.
> +	 * Here hugetlb pages will always guarantee the anon-exclusiveness
> +	 * of the head page represents the tail pages.
> +	 */
> +	if (PageHuge(page) && !PageHead(page))
> +		page = compound_head(page);
>  	return test_bit(PG_anon_exclusive, &PF_ANY(page, 1)->flags);
>  }
>  
> diff --git a/mm/internal.h b/mm/internal.h
> index 9512de7398d5..87f6e4fd56a5 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1259,16 +1259,6 @@ static inline bool gup_must_unshare(struct vm_area_struct *vma,
>  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
>  		smp_rmb();
>  
> -	/*
> -	 * During GUP-fast we might not get called on the head page for a
> -	 * hugetlb page that is mapped using cont-PTE, because GUP-fast does
> -	 * not work with the abstracted hugetlb PTEs that always point at the
> -	 * head page. For hugetlb, PageAnonExclusive only applies on the head
> -	 * page (as it cannot be partially COW-shared), so lookup the head page.
> -	 */
> -	if (unlikely(!PageHead(page) && PageHuge(page)))
> -		page = compound_head(page);
> -
>  	/*
>  	 * Note that PageKsm() pages cannot be exclusive, and consequently,
>  	 * cannot get pinned.