Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2024-02-07 12:20:27
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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:quoted
On Sat, Jan 27, 2024 at 01:04:15PM +0800, Nanyong Sun wrote:quoted
On 2024/1/26 2:06, Catalin Marinas wrote:quoted
On Sat, Jan 13, 2024 at 05:44:33PM +0800, Nanyong Sun wrote:quoted
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. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel