Re: [PATCH v15 00/23] Generic page walk and ptdump

From: Thomas Hellstrom <thellstrom@vmware.com>
To: Steven Price <steven.price@arm.com>,
	David Hildenbrand <david@redhat.com>
Cc: "Qian Cai" <cai@lca.pw>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	"Mark Rutland" <Mark.Rutland@arm.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Catalin Marinas" <Catalin.Marinas@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Andy Lutomirski" <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"James Morse" <James.Morse@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Will Deacon" <will@kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"Liang, Kan" <kan.liang@linux.intel.com>
Subject: Re: [PATCH v15 00/23] Generic page walk and ptdump
Date: Wed, 4 Dec 2019 17:51:06 +0000	[thread overview]
Message-ID: <MN2PR05MB61416E0CF8A99C91CD69FC43A15D0@MN2PR05MB6141.namprd05.prod.outlook.com> (raw)
In-Reply-To: 20191204163235.GA1597@arm.com

On 12/4/19 5:32 PM, Steven Price wrote:
> On Wed, Dec 04, 2019 at 02:56:58PM +0000, David Hildenbrand wrote:
>> On 04.12.19 15:54, Qian Cai wrote:
>>>
>>>> On Dec 3, 2019, at 6:02 AM, David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 06.11.19 16:05, Steven Price wrote:
>>>>> On 06/11/2019 13:31, Qian Cai wrote:
>>>>>>
>>>>>>> On Nov 4, 2019, at 2:35 PM, Qian Cai <cai@lca.pw> wrote:
>>>>>>>
>>>>>>> On Fri, 2019-11-01 at 14:09 +0000, Steven Price wrote:
>>>>> [...]
>>>>>>>> Changes since v14:
>>>>>>>> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20191028135910.33253-1-steven.price%40arm.com%2F&amp;data=02%7C01%7Cthellstrom%40vmware.com%7C9f50ca595f81432eff5b08d778d7968a%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C1%7C637110739727088799&amp;sdata=B3n6TFU7hluQyAXUOEaHBAGNC8mhscMfxSJi%2FrFr%2Flo%3D&amp;reserved=0
>>>>>>>> * Switch walk_page_range() into two functions, the existing
>>>>>>>>   walk_page_range() now still requires VMAs (and treats areas without a
>>>>>>>>   VMA as a 'hole'). The new walk_page_range_novma() ignores VMAs and
>>>>>>>>   will report the actual page table layout. This fixes the previous
>>>>>>>>   breakage of /proc/<pid>/pagemap
>>>>>>>> * New patch at the end of the series which reduces the 'level' numbers
>>>>>>>>   by 1 to simplify the code slightly
>>>>>>>> * Added tags
>>>>>>> Does this new version also take care of this boot crash seen with v14? Suppose
>>>>>>> it is now breaking CONFIG_EFI_PGT_DUMP=y? The full config is,
>>>>>>>
>>>>>>> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fraw.githubusercontent.com%2Fcailca%2Flinux-mm%2Fmaster%2Fx86.config&amp;data=02%7C01%7Cthellstrom%40vmware.com%7C9f50ca595f81432eff5b08d778d7968a%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C1%7C637110739727088799&amp;sdata=ymVh49kh7VL9yseRdkjSbTwRh%2B7yBXxhK7QMTUzwn4U%3D&amp;reserved=0
>>>>>>>
>>>>>> V15 is indeed DOA here.
>>>>> Thanks for finding this, it looks like EFI causes issues here. The below fixes
>>>>> this for me (booting in QEMU).
>>>>>
>>>>> Andrew: do you want me to send out the entire series again for this fix, or
>>>>> can you squash this into mm-pagewalk-allow-walking-without-vma.patch?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Steve
>>>>>
>>>>> ---8<---
>>>>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>>>>> index c7529dc4f82b..70dcaa23598f 100644
>>>>> --- a/mm/pagewalk.c
>>>>> +++ b/mm/pagewalk.c
>>>>> @@ -90,7 +90,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>>>>>  			split_huge_pmd(walk->vma, pmd, addr);
>>>>>  			if (pmd_trans_unstable(pmd))
>>>>>  				goto again;
>>>>> -		} else if (pmd_leaf(*pmd)) {
>>>>> +		} else if (pmd_leaf(*pmd) || !pmd_present(*pmd)) {
>>>>>  			continue;
>>>>>  		}
>>>>>
>>>>> @@ -141,7 +141,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>>>>  			split_huge_pud(walk->vma, pud, addr);
>>>>>  			if (pud_none(*pud))
>>>>>  				goto again;
>>>>> -		} else if (pud_leaf(*pud)) {
>>>>> +		} else if (pud_leaf(*pud) || !pud_present(*pud)) {
>>>>>  			continue;
>>>>>  		}
>>>>>
>>>>>
>>>> Even with this fix, booting for me under QEMU fails. See
>>>>
>>>> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flinux-mm%2Fb7ce62f2-9a48-6e48-6685-003431e521aa%40redhat.com%2F&amp;data=02%7C01%7Cthellstrom%40vmware.com%7C9f50ca595f81432eff5b08d778d7968a%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C1%7C637110739727088799&amp;sdata=fRuLrmrzNEkU2MFzSVdyVyXyRoyZ95yZOYuy7aMSi7A%3D&amp;reserved=0
>>>>
>>> Yes, for some reasons, this starts to crash on almost all arches here, so it might be worth
>>> for Andrew to revert those in the meantime while allowing Steven to rework.
>> I agree, this produces too much noise.
> I've bisected this problem and it's a merge conflict with:
>
> ace88f1018b8 ("mm: pagewalk: Take the pagetable lock in walk_pte_range()")
>
> Reverting that commit "fixes" the problem. That commit adds a call to
> pte_offset_map_lock(), however that isn't necessarily safe when
> considering an "unusual" mapping in the kernel. Combined with my patch
> set this leads to the BUG when walking the kernel's page tables.
>
> At this stage I think it's best if Andrew drops my series and I'll try
> to rework it on top -rc1 fixing up this conflict and the other x86
> 32-bit issue that has cropped up.

Hi,

Unfortunately I wasn't aware of that conflict.

Perhaps something similar to this

https://elixir.bootlin.com/linux/v5.4/source/mm/memory.c#L2012

would fix at least this particular issue?

/Thomas

>
> Steve
>