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

Ryan Roberts
2024-04-02T16:18:57Z

On 02/04/2024 17:00, Matthew Wilcox wrote:
> On Tue, Apr 02, 2024 at 05:26:28PM +0200, David Hildenbrand wrote:
>>> The oops trigger is at mm/gup.c:778:
>>> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
>>>
>>> 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.
>>
>> 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).
>
> That was a practical limitation on my part. We have various parts of
> the MM which assume that pmd_page() returns a head page and until we
> get all of those fixed, adding support for folios larger than PMD_SIZE
> was only going to cause trouble for no significant wins.
>
> I agree with you we should get rid of this assertion entirely. We should
> fix all the places which assume that pmd_page() returns a head page,
> but that may take some time.
>
> As an example, filemap_map_pmd() has:
>
> if (pmd_none(*vmf->pmd) && folio_test_pmd_mappable(folio)) {
> struct page *page = folio_file_page(folio, start);
> vm_fault_t ret = do_set_pmd(vmf, page);
>
> and then do_set_pmd() has:
>
> if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER)
> return ret;
>
> so we'd simply refuse to use a PMD to map a folio larger than PMD_SIZE.
> There's a lot of work to be done to make this work generally (not to
> mention figuring out how to handle mapcount for such folios ;-).
>
> This particular case seems straightforward though. Just remove the
> assertion.

Removing the assertion gets me further, but then I end up with this:

[ 9.748422] kernel BUG at include/linux/page-flags.h:1098!
[ 9.748897] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
[ 9.749590] Modules linked in:
[ 9.749867] CPU: 2 PID: 1155 Comm: gup_longterm Not tainted 6.9.0-rc2-00210-g910ff1a347e4-dirty #12
[ 9.750682] Hardware name: linux,dummy-virt (DT)
[ 9.751095] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 9.751729] pc : follow_page_mask+0x730/0x850
[ 9.752152] lr : follow_page_mask+0x730/0x850
[ 9.752573] sp : ffff8000898f3aa0
[ 9.752882] x29: ffff8000898f3aa0 x28: fffffdffc52b91a8 x27: 0000000000000001
[ 9.753543] x26: ffff00014ae46d08 x25: 00003c0005d88000 x24: fffffdffc5d88000
[ 9.754221] x23: ffffc1ffc0000000 x22: 0000000000080101 x21: ffff8000898f3ba8
[ 9.754875] x20: 0000fffff4200000 x19: ffff0001a3d64450 x18: 0000000000000010
[ 9.755567] x17: 2864616548656761 x16: 5021202626202965 x15: 6761702865677548
[ 9.756254] x14: 6567615028454741 x13: 2929656761702864 x12: 6165486567615021
[ 9.756953] x11: 2026262029656761 x10: ffffaaac08f1d6e0 x9 : ffffaaac0612f090
[ 9.757671] x8 : 00000000ffffefff x7 : ffffaaac08f1d6e0 x6 : 0000000000000000
[ 9.758356] x5 : ffff00017ffb9cc8 x4 : 0000000000000fff x3 : 0000000000000000
[ 9.758983] x2 : 0000000000000000 x1 : ffff000189ecb480 x0 : 0000000000000046
[ 9.759663] Call trace:
[ 9.759901] follow_page_mask+0x730/0x850
[ 9.760293] __get_user_pages+0xf4/0x3e8
[ 9.760683] __gup_longterm_locked+0x204/0xa70
[ 9.761110] pin_user_pages+0x88/0xc0
[ 9.761486] gup_test_ioctl+0x860/0xc40
[ 9.761866] __arm64_sys_ioctl+0xb0/0x100
[ 9.762254] invoke_syscall+0x50/0x128
[ 9.762630] el0_svc_common.constprop.0+0x48/0xf8
[ 9.763104] do_el0_svc+0x28/0x40
[ 9.763413] el0_svc+0x34/0xe0
[ 9.763699] el0t_64_sync_handler+0x13c/0x158
[ 9.764139] el0t_64_sync+0x190/0x198
[ 9.764465] Code: aa1803e0 d000d8e1 911d6021 97fff4c9 (d4210000)
[ 9.765053] ---[ end trace 0000000000000000 ]---
[ 9.765520] note: gup_longterm[1155] exited with irqs disabled
[ 9.766146] note: gup_longterm[1155] exited with preempt_count 2
[ 9.767366] ------------[ cut here ]------------
[ 9.768062] WARNING: CPU: 2 PID: 0 at kernel/context_tracking.c:128 ct_kernel_exit.constprop.0+0x108/0x120
[ 9.769146] Modules linked in:
[ 9.769429] CPU: 2 PID: 0 Comm: swapper/2 Tainted: G D 6.9.0-rc2-00210-g910ff1a347e4-dirty #12
[ 9.770338] Hardware name: linux,dummy-virt (DT)
[ 9.770837] pstate: 204003c5 (nzCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 9.771615] pc : ct_kernel_exit.constprop.0+0x108/0x120
[ 9.772150] lr : ct_idle_enter+0x10/0x20
[ 9.772539] sp : ffff8000801b3dc0
[ 9.772913] x29: ffff8000801b3dc0 x28: 0000000000000000 x27: 0000000000000000
[ 9.773769] x26: 0000000000000000 x25: ffff00014149e900 x24: 0000000000000000
[ 9.774526] x23: 0000000000000000 x22: ffffaaac08e99d48 x21: ffffaaac08385730
[ 9.775255] x20: ffffaaac08e99c28 x19: ffff00017ffc8da0 x18: 0000fffff5ffffff
[ 9.775924] x17: 0000000000000000 x16: 1fffe0002a57c9e1 x15: 0000000000000001
[ 9.776619] x14: ffffffffffffffff x13: 0000000000000000 x12: ffffaaac07a06968
[ 9.777246] x11: 000000ae44c42eec x10: 0000000000000ad0 x9 : ffffaaac06189230
[ 9.777942] x8 : ffff00014149f430 x7 : 02c9acb509db422c x6 : 000000001015a9f0
[ 9.778635] x5 : 4000000000000002 x4 : ffff555577c46000 x3 : ffff8000801b3dc0
[ 9.779671] x2 : ffffaaac08382da0 x1 : 4000000000000000 x0 : ffffaaac08382da0
[ 9.780703] Call trace:
[ 9.781150] ct_kernel_exit.constprop.0+0x108/0x120
[ 9.781949] ct_idle_enter+0x10/0x20
[ 9.782246] default_idle_call+0x3c/0x160
[ 9.782624] do_idle+0x21c/0x280
[ 9.782945] cpu_startup_entry+0x3c/0x50
[ 9.783268] secondary_start_kernel+0x140/0x168
[ 9.783818] __secondary_switched+0xb8/0xc0
[ 9.784163] ---[ end trace 0000000000000000 ]---

Which is caused by this:

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); <<<<
return test_bit(PG_anon_exclusive, &PF_ANY(page, 1)->flags);
}

Which is called from can_follow_write_pmd(), called just after the assert I just commented out.

It's triggered by this test:

# [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (32768 kB)

Which is the first MAP_PRIVATE test for cont-pmd mapped hugetlb. (All MAP_SHARED tests are passing).

Looks like can_follow_write_pmd() returns early for VM_SHARED mappings.

I don't think we only keep the PAE flag in the head page for hugetlb pages? So we can't just remove this assert?

I tried just commenting it out and get assert further down follow_huge_pmd():

VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
!PageAnonExclusive(page), page);

Thanks,
Ryan