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

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Message-ID: <CA+EHjTxymfya75KdOrUsSUhtfmxe180DedhJpLQAGeCjsum_nw@mail.gmail.com>
Date: Tue, 2 Sep 2025 09:50:43 +0100
From: Fuad Tabba <tabba@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: "Roy, Patrick" <roypat@...zon.co.uk>, "ackerleytng@...gle.com" <ackerleytng@...gle.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>, "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 Tue, 2 Sept 2025 at 09:46, David Hildenbrand <david@...hat.com> wrote:
>
> On 02.09.25 09:59, Fuad Tabba wrote:
> > Hi Patrick,
> >
> > On Mon, 1 Sept 2025 at 15:56, Roy, Patrick <roypat@...zon.co.uk> wrote:
> >>
> >> 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
> >
> > I don't have any strong preference regarding which way, as long as
> > it's consistent. The thing that should be avoided is having one
> > function with a parameter marked as const, pass that parameter (or
> > something derived from it), to a non-const function.
>
> I think the compiler will tell you that that is not ok (and you'd have
> to force-cast the const it away).

Not for the scenario I'm worried about. The compiler didn't complain
about this (from this patch):

+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);
+}

vma_is_no_direct_map() takes a const, but mapping_no_direct_map()
doesn't. For now, mapping_no_direct_map() doesn't modify anything. But
it could, and the compiler wouldn't complain.

Cheers,
/fuad


> Agreed that we should be using const * for these simple getter/test
> functions.
>
> --
> Cheers
>
> David / dhildenb
>