[PATCH v8 36/43] arm64: mm: Add support for folding PUDs at runtime
Ryan Roberts
ryan.roberts at arm.com
Fri Mar 1 01:47:33 PST 2024
On 01/03/2024 09:37, Ard Biesheuvel wrote:
> On Fri, 1 Mar 2024 at 10:10, Ard Biesheuvel <ardb at kernel.org> wrote:
>>
>> On Fri, 1 Mar 2024 at 09:54, Ryan Roberts <ryan.roberts at arm.com> wrote:
>>>
>>> + Matthew
>>>
>>>
>>> On 29/02/2024 23:01, Nathan Chancellor wrote:
>>>> On Thu, Feb 29, 2024 at 02:17:52PM +0000, Ryan Roberts wrote:
>>>>> Hi Ard,
>>>>>
>>>>> On 14/02/2024 12:29, Ard Biesheuvel wrote:
>>>>>> From: Ard Biesheuvel <ardb at kernel.org>
>>>>>>
>>>>>> In order to support LPA2 on 16k pages in a way that permits non-LPA2
>>>>>> systems to run the same kernel image, we have to be able to fall back to
>>>>>> at most 48 bits of virtual addressing.
>>>>>>
>>>>>> Falling back to 48 bits would result in a level 0 with only 2 entries,
>>>>>> which is suboptimal in terms of TLB utilization. So instead, let's fall
>>>>>> back to 47 bits in that case. This means we need to be able to fold PUDs
>>>>>> dynamically, similar to how we fold P4Ds for 48 bit virtual addressing
>>>>>> on LPA2 with 4k pages.
>>>>>
>>>>> I'm seeing a panic during boot in today's linux-next (20240229) and bisect seems pretty confident that this commit is the offender. That said, its the merge commit that shows up as the problem commit:
>>>>>
>>>>> 26843fe8fa72 Merge branch 'for-next/core' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux
>>>>>
>>>>> but when testing the arm64's for-next/core, the problem doesn't exist. So I rebased the branch into linux-next and bisected again. That time, it fingers this patch. So I guess there is some interaction between this and other changes in next?
>>>> <...>
>>>>> [ 0.161062] debug_vm_pgtable: [debug_vm_pgtable ]: Validating architecture page table helpers
>>>>> [ 0.161416] BUG: Bad page state in process swapper/0 pfn:18a65d
>>>>> [ 0.161634] page does not match folio
>>>>> [ 0.161753] page: refcount:0 mapcount:-512 mapping:0000000000000000 index:0x0 pfn:0x18a65d
>>>>> [ 0.162046] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000008
>>>>> [ 0.162332] Mem abort info:
>>>>> [ 0.162427] ESR = 0x0000000096000004
>>>>> [ 0.162559] EC = 0x25: DABT (current EL), IL = 32 bits
>>>>> [ 0.162723] SET = 0, FnV = 0
>>>>> [ 0.162827] EA = 0, S1PTW = 0
>>>>> [ 0.162933] FSC = 0x04: level 0 translation fault
>>>>> [ 0.163089] Data abort info:
>>>>> [ 0.163189] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
>>>>> [ 0.163370] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
>>>>> [ 0.163539] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
>>>>> [ 0.163719] [0000000000000008] user address but active_mm is swapper
>>>>> [ 0.163934] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
>>>>> [ 0.164143] Modules linked in:
>>>>> [ 0.164251] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 6.8.0-rc6-00966-gde701dc1f7f8 #25
>>>>> [ 0.164516] Hardware name: linux,dummy-virt (DT)
>>>>> [ 0.164704] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
>>>>> [ 0.165052] pc : get_pfnblock_flags_mask+0x3c/0x68
>>>>> [ 0.165281] lr : __dump_page+0x1a0/0x408
>>>>> [ 0.165504] sp : ffff80008007b8f0
>>>>> [ 0.165715] x29: ffff80008007b8f0 x28: 0000000000ffffc0 x27: 0000000000000000
>>>>> [ 0.166047] x26: ffff80008007b950 x25: 0000000000000000 x24: 00000000fffffdff
>>>>> [ 0.166358] x23: ffffba8a417ba000 x22: 000000000018a65d x21: ffffba8a41601bf8
>>>>> [ 0.166701] x20: ffff80008007b950 x19: ffff80008007b950 x18: 0000000000000006
>>>>> [ 0.167036] x17: 78303a7865646e69 x16: 2030303030303030 x15: 0720072007200720
>>>>> [ 0.167365] x14: 0720072007200720 x13: 0720072007200720 x12: 0720072007200720
>>>>> [ 0.167693] x11: 0720072007200720 x10: ffffba8a4269c038 x9 : ffffba8a3fb0d0b8
>>>>> [ 0.168017] x8 : 00000000ffffefff x7 : ffffba8a4269c038 x6 : 80000000fffff000
>>>>> [ 0.168346] x5 : 000003fffff81de4 x4 : 0001fffffc0ef230 x3 : 0000000000000000
>>>>> [ 0.168699] x2 : 0000000000000007 x1 : fffffe0779181ee5 x0 : 00000000001fffff
>>>>> [ 0.169041] Call trace:
>>>>> [ 0.169164] get_pfnblock_flags_mask+0x3c/0x68
>>>>> [ 0.169413] dump_page+0x2c/0x70
>>>>> [ 0.169565] bad_page+0x84/0x130
>>>>> [ 0.169734] free_page_is_bad_report+0xa0/0xb8
>>>>> [ 0.169958] free_unref_page_prepare+0x350/0x428
>>>>> [ 0.170132] free_unref_page+0x50/0x1f0
>>>>> [ 0.170278] __free_pages+0x11c/0x160
>>>>> [ 0.170417] free_pages.part.0+0x6c/0x88
>>>>> [ 0.170576] free_pages+0x1c/0x38
>>>>> [ 0.170703] destroy_args+0x1c8/0x330
>>>>> [ 0.170890] debug_vm_pgtable+0xae8/0x10f8
>>>>> [ 0.171059] do_one_initcall+0x60/0x2c0
>>>>> [ 0.171222] kernel_init_freeable+0x1ec/0x3d8
>>>>> [ 0.171406] kernel_init+0x28/0x1f0
>>>>> [ 0.171557] ret_from_fork+0x10/0x20
>>>>> [ 0.171712] Code: d37b1884 f100007f 8b040064 9a831083 (f9400460)
>>>>> [ 0.171963] ---[ end trace 0000000000000000 ]---
>>>>> [ 0.172156] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
>>>>> [ 0.172383] SMP: stopping secondary CPUs
>>>>> [ 0.172649] Kernel Offset: 0x3a89bf800000 from 0xffff800080000000
>>>>> [ 0.173923] PHYS_OFFSET: 0xfffff76180000000
>>>>> [ 0.174585] CPU features: 0x0,00000000,2004454a,13867723
>>>>> [ 0.175707] Memory Limit: none
>>>>> [ 0.176261] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---
>>>>
>>>> I did a second bisection by merging https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-next/stage1-lpa2
>>>> on top of the merges before for-next/core and eventually landed on:
>>>>
>>>> d67cd9f23139ddfd7e0ef1e18474c16445188433 is the first bad commit
>>>> commit d67cd9f23139ddfd7e0ef1e18474c16445188433
>>>> Author: Matthew Wilcox (Oracle) <willy at infradead.org>
>>>> Date: Tue Feb 27 19:23:31 2024 +0000
>>>>
>>>> mm: add __dump_folio()
>>>>
>>>> Turn __dump_page() into a wrapper around __dump_folio(). Snapshot the
>>>> page & folio into a stack variable so we don't hit BUG_ON() if an
>>>> allocation is freed under us and what was a folio pointer becomes a
>>>> pointer to a tail page.
>>>>
>>>> Link: https://lkml.kernel.org/r/20240227192337.757313-5-willy@infradead.org
>>>> Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
>>>> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
>>>
>>> So is that suggesting that Ard's patch is doing something that the old
>>> __dump_page() was ok with but the new version doesn't like? I don't think so,
>>> because the bad page detection has already happened before we get to __dump_page().
>>>
>>
>> Yes, there are clearly two different issues at play here. The NULL
>> dereference might be an issue in the __dump_page() patch, but going
>> down that code path in the first place seems like it might be a
>> problem with mine.
>>
>> The mapcount of -512 looks interesting as well.
>>
>>> So I'm not really sure how this patch is involved? I'm hoping that either Ard or
>>> Matthew may be able to take a look and advise.
>>>
>>
>
> The crash does not reproduce for me, but the warning can be fixed by
>
> diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
> index aeba2cf15a25..78f30b782889 100644
> --- a/arch/arm64/include/asm/pgalloc.h
> +++ b/arch/arm64/include/asm/pgalloc.h
> @@ -61,7 +61,7 @@ static inline void pud_free(struct mm_struct *mm, pud_t *pud)
> if (!pgtable_l4_enabled())
> return;
> BUG_ON((unsigned long)pud & (PAGE_SIZE-1));
> - free_page((unsigned long)pud);
> + __pud_free(mm, pud);
> }
> #else
> static inline void __p4d_populate(p4d_t *p4dp, phys_addr_t pudp, p4dval_t prot)
>
> I'll send this out as a patch shortly.
Great thanks! I'm pretty sure I've found the bug in Matthew's patch - it is
copying the struct page to the stack to avoid a potential race, but later some
macros are hiding a page_to_pfn(). Since the page's address is on the stack, I
reckon that's giving a bogus pfn. Just confirming and writing it up and will
send against the original patch shortly.
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