linux-kernel - Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT

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Message-ID: <20250901145632.28172-1-roypat@amazon.co.uk>
Date: Mon, 1 Sep 2025 14:56:33 +0000
From: "Roy, Patrick" <roypat@...zon.co.uk>
To: "Roy, Patrick" <roypat@...zon.co.uk>
CC: "ackerleytng@...gle.com" <ackerleytng@...gle.com>, "david@...hat.com"
	<david@...hat.com>, "Manwaring, Derek" <derekmn@...zon.com>, "Thomson, Jack"
	<jackabt@...zon.co.uk>, "Kalyazin, Nikita" <kalyazin@...zon.co.uk>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "kvmarm@...ts.linux.dev"
	<kvmarm@...ts.linux.dev>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "rppt@...nel.org"
	<rppt@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"tabba@...gle.com" <tabba@...gle.com>, "vbabka@...e.cz" <vbabka@...e.cz>,
	"will@...nel.org" <will@...nel.org>, "Cali, Marco" <xmarcalx@...zon.co.uk>
Subject: Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT_MAP

On Mon, 2025-09-01 at 14:54 +0100, "Roy, Patrick" wrote:
> 
> Hi Fuad!
> 
> On Thu, 2025-08-28 at 11:21 +0100, Fuad Tabba wrote:
>> Hi Patrick,
>>
>> On Thu, 28 Aug 2025 at 10:39, Roy, Patrick <roypat@...zon.co.uk> wrote:
>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>>> index 12a12dae727d..b52b28ae4636 100644
>>> --- a/include/linux/pagemap.h
>>> +++ b/include/linux/pagemap.h
>>> @@ -211,6 +211,7 @@ enum mapping_flags {
>>>                                    folio contents */
>>>         AS_INACCESSIBLE = 8,    /* Do not attempt direct R/W access to the mapping */
>>>         AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
>>> +       AS_NO_DIRECT_MAP = 10,  /* Folios in the mapping are not in the direct map */
>>>         /* Bits 16-25 are used for FOLIO_ORDER */
>>>         AS_FOLIO_ORDER_BITS = 5,
>>>         AS_FOLIO_ORDER_MIN = 16,
>>> @@ -346,6 +347,21 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(struct address_spac
>>>         return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
>>>  }
>>>
>>> +static inline void mapping_set_no_direct_map(struct address_space *mapping)
>>> +{
>>> +       set_bit(AS_NO_DIRECT_MAP, &mapping->flags);
>>> +}
>>> +
>>> +static inline bool mapping_no_direct_map(struct address_space *mapping)
>>> +{
>>> +       return test_bit(AS_NO_DIRECT_MAP, &mapping->flags);
>>> +}
>>> +
>>> +static inline bool vma_is_no_direct_map(const struct vm_area_struct *vma)
>>> +{
>>> +       return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping);
>>> +}
>>> +
>> Any reason vma is const whereas mapping in the function that it calls
>> (defined above it) isn't?
> 
> Ah, I cannot say that that was a conscious decision, but rather an artifact of
> the code that I looked at for reference when writing these two simply did it
> this way.  Are you saying both should be const, or neither (in my mind, both
> could be const, but the mapping_*() family of functions further up in this file
> dont take const arguments, so I'm a bit unsure now)?

Hah, just saw
https://lore.kernel.org/linux-mm/20250901123028.3383461-3-max.kellermann@ionos.com/.
Guess that means "both should be const" then :D

>> Cheers,
>> /fuad
> 
> Best,
> Patrick