linux-kernel - Re: [RFC 1/2] cma: Fix CMA_MIN_ALIGNMENT_BYTES during early

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Message-ID: <83eb128e-4f06-4725-a843-a4563f246a44@redhat.com>
Date: Tue, 8 Oct 2024 15:50:46 +0200
From: David Hildenbrand <david@...hat.com>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 linuxppc-dev@...ts.ozlabs.org
Cc: linux-mm@...ck.org, Sourabh Jain <sourabhjain@...ux.ibm.com>,
 Hari Bathini <hbathini@...ux.ibm.com>, Zi Yan <ziy@...dia.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
 Donet Tom <donettom@...ux.vnet.ibm.com>, LKML
 <linux-kernel@...r.kernel.org>, Sachin P Bappalige <sachinpb@...ux.ibm.com>
Subject: Re: [RFC 1/2] cma: Fix CMA_MIN_ALIGNMENT_BYTES during early_init

On 08.10.24 15:27, Ritesh Harjani (IBM) wrote:
> During early init CMA_MIN_ALIGNMENT_BYTES can be PAGE_SIZE,
> since pageblock_order is still zero and it gets initialized
> later during paging_init() e.g.
> paging_init() -> free_area_init() -> set_pageblock_order().
> 
> One such use case is -
> early_setup() -> early_init_devtree() -> fadump_reserve_mem()
> 
> This causes CMA memory alignment check to be bypassed in
> cma_init_reserved_mem(). Then later cma_activate_area() can hit
> a VM_BUG_ON_PAGE(pfn & ((1 << order) - 1)) if the reserved memory
> area was not pageblock_order aligned.
> 
> Instead of fixing it locally for fadump case on PowerPC, I believe
> this should be fixed for CMA_MIN_ALIGNMENT_BYTES.

I think we should add a way to catch the usage of 
CMA_MIN_ALIGNMENT_BYTES before it actually has meaning (before 
pageblock_order was set) and fix the PowerPC usage by reshuffling the 
code accordingly.

-- 
Cheers,

David / dhildenb