linux-kernel - Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize

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Date: Thu, 8 Feb 2024 17:44:48 +0800
From: Nanyong Sun <sunnanyong@...wei.com>
To: Catalin Marinas <catalin.marinas@....com>, Matthew Wilcox
	<willy@...radead.org>
CC: Will Deacon <will@...nel.org>, <mike.kravetz@...cle.com>,
	<muchun.song@...ux.dev>, <akpm@...ux-foundation.org>,
	<anshuman.khandual@....com>, <wangkefeng.wang@...wei.com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>
Subject: Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize


在 2024/2/7 20:20, Catalin Marinas 写道:
> On Wed, Feb 07, 2024 at 11:21:17AM +0000, Matthew Wilcox wrote:
>> On Wed, Feb 07, 2024 at 11:12:52AM +0000, Will Deacon wrote:
>>> On Sat, Jan 27, 2024 at 01:04:15PM +0800, Nanyong Sun wrote:
>>>> On 2024/1/26 2:06, Catalin Marinas wrote:
>>>>> On Sat, Jan 13, 2024 at 05:44:33PM +0800, Nanyong Sun wrote:
>>>>>> HVO was previously disabled on arm64 [1] due to the lack of necessary
>>>>>> BBM(break-before-make) logic when changing page tables.
>>>>>> This set of patches fix this by adding necessary BBM sequence when
>>>>>> changing page table, and supporting vmemmap page fault handling to
>>>>>> fixup kernel address translation fault if vmemmap is concurrently accessed.
>>>>> I'm not keen on this approach. I'm not even sure it's safe. In the
>>>>> second patch, you take the init_mm.page_table_lock on the fault path but
>>>>> are we sure this is unlocked when the fault was taken?
>>>> I think this situation is impossible. In the implementation of the second
>>>> patch, when the page table is being corrupted
>>>> (the time window when a page fault may occur), vmemmap_update_pte() already
>>>> holds the init_mm.page_table_lock,
>>>> and unlock it until page table update is done.Another thread could not hold
>>>> the init_mm.page_table_lock and
>>>> also trigger a page fault at the same time.
>>>> If I have missed any points in my thinking, please correct me. Thank you.
>>> It still strikes me as incredibly fragile to handle the fault and trying
>>> to reason about all the users of 'struct page' is impossible. For example,
>>> can the fault happen from irq context?
>> The pte lock cannot be taken in irq context (which I think is what
>> you're asking?)
> With this patchset, I think it can: IRQ -> interrupt handler accesses
> vmemmap -> faults -> fault handler in patch 2 takes the
> init_mm.page_table_lock to wait for the vmemmap rewriting to complete.
> Maybe it works if the hugetlb code disabled the IRQs but, as Will said,
> such fault in any kernel context looks fragile.
How about take a new lock with irq disabled during BBM, like:

+void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte)
+{
+    spin_lock_irq(NEW_LOCK);
+    pte_clear(&init_mm, addr, ptep);
+    flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+    set_pte_at(&init_mm, addr, ptep, pte);
+    spin_unlock_irq(NEW_LOCK);
+}