linux-kernel - Re: [PATCH v1 15/16] mm: Generalize arch_sync_kernel

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Message-ID: <67bcb2f2-8cc1-43e3-b5cc-6c8ef5da8a95@arm.com>
Date: Thu, 13 Feb 2025 09:17:21 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Anshuman Khandual <anshuman.khandual@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>,
 Pasha Tatashin <pasha.tatashin@...een.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
 Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Dev Jain <dev.jain@....com>, Alexandre Ghiti <alexghiti@...osinc.com>,
 Steve Capper <steve.capper@...aro.org>, Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 15/16] mm: Generalize arch_sync_kernel_mappings()

>>>> +/**
>>>> + * arch_update_kernel_mappings_end - A batch of kernel pgtable mappings have
>>>> + * been updated.
>>>> + * @start: Virtual address of start of range that was updated.
>>>> + * @end: Virtual address of end of range that was updated.
>>>> + *
>>>> + * An optional hook to inform architecture code that a batch update is complete.
>>>> + * This balances a previous call to arch_update_kernel_mappings_begin().
>>>> + *
>>>> + * An architecture may override this for any purpose, such as exiting a lazy
>>>> + * mode previously entered with arch_update_kernel_mappings_begin() or syncing
>>>> + * kernel mappings to a secondary pgtable. The default implementation calls an
>>>> + * arch-provided arch_sync_kernel_mappings() if any arch-defined pgtable level
>>>> + * was updated.
>>>> + *
>>>> + * Context: Called in task context and may be preemptible.
>>>> + */
>>>> +static inline void arch_update_kernel_mappings_end(unsigned long start,
>>>> +						   unsigned long end,
>>>> +						   pgtbl_mod_mask mask)
>>>> +{
>>>> +	if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
>>>> +		arch_sync_kernel_mappings(start, end);
>>>> +}
>>>
>>> One arch call back calling yet another arch call back sounds bit odd. 
>>
>> It's no different from the default implementation of arch_make_huge_pte()
>> calling pte_mkhuge() is it?
> 
> Agreed. arch_make_huge_pte() ---> pte_mkhuge() where either helpers can be
> customized in the platform is another such example but unless necessary we
> should probably avoid following that. Anyways it's not a big deal I guess.
> 
>>
>>> Also
>>> should not ARCH_PAGE_TABLE_SYNC_MASK be checked both for __begin and __end
>>> callbacks in case a platform subscribes into this framework. 
>>
>> I'm not sure how that would work? The mask is accumulated during the pgtable
>> walk. So we don't have a mask until we get to the end.
> 
> A non-zero ARCH_PAGE_TABLE_SYNC_MASK indicates that a platform is subscribing
> to this mechanism. So could ARCH_PAGE_TABLE_SYNC_MASK != 0 be used instead ?

There are now 2 levels of mechanism:

Either: arch defines ARCH_PAGE_TABLE_SYNC_MASK to be non-zero and provides
arch_sync_kernel_mappings(). This is unchanged from how it was before.

Or: arch defines it's own version of one or both of
arch_update_kernel_mappings_begin() and arch_update_kernel_mappings_end().

So a non-zero ARCH_PAGE_TABLE_SYNC_MASK indicates that a platform is subscribing
to the *first* mechanism. It has nothing to do with the second mechanism. If the
platform defines arch_update_kernel_mappings_begin() it wants it to be called.
If it doesn't define it, then it doesn't get called.

Thanks,
Ryan