linux-kernel - Re: [PATCH v1 02/11] mm: convert track_pfn_insert() to pfnmap_sanitize

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Message-ID: <75998f7c-93d2-4b98-bb53-8d858b2c108e@redhat.com>
Date: Mon, 28 Apr 2025 16:58:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 linux-trace-kernel@...r.kernel.org, Dave Hansen
 <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 "H. Peter Anvin" <hpa@...or.com>, Jani Nikula <jani.nikula@...ux.intel.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>, Tvrtko Ursulin
 <tursulin@...ulin.net>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Andrew Morton <akpm@...ux-foundation.org>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH v1 02/11] mm: convert track_pfn_insert() to
 pfnmap_sanitize_pgprot()

>> What it does on PAT (only implementation so far ...) is looking up the
>> memory type to select the caching mode that can be use.
>>
>> "sanitize" was IMHO a good fit, because we must make sure that we don't use
>> the wrong caching mode.
>>
>> update/setup/... don't make that quite clear. Any other suggestions?
> 
> I'm very poor on naming.. :( So far anything seems slightly better than
> sanitize to me, as the word "sanitize" is actually also used in memtype.c
> for other purpose.. see sanitize_phys().

Sure, one can sanitize a lot of things. Here it's the cachemode/pgrpot, 
in the other functions it's an address.

Likely we should just call it pfnmap_X_cachemode()/

Set/update don't really fit for X in case pfnmap_X_cachemode() is a NOP.

pfnmap_setup_cachemode() ? Hm.

> 
>>
>>>
>>>> + * @pfn: the start of the pfn range
>>>> + * @size: the size of the pfn range
>>>> + * @prot: the pgprot to sanitize
>>>> + *
>>>> + * Sanitize the given pgprot for a pfn range, for example, adjusting the
>>>> + * cachemode.
>>>> + *
>>>> + * This function cannot fail for a single page, but can fail for multiple
>>>> + * pages.
>>>> + *
>>>> + * Returns 0 on success and -EINVAL on error.
>>>> + */
>>>> +int pfnmap_sanitize_pgprot(unsigned long pfn, unsigned long size,
>>>> +		pgprot_t *prot);
>>>>    extern int track_pfn_copy(struct vm_area_struct *dst_vma,
>>>>    		struct vm_area_struct *src_vma, unsigned long *pfn);
>>>>    extern void untrack_pfn_copy(struct vm_area_struct *dst_vma,
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index fdcf0a6049b9f..b8ae5e1493315 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -1455,7 +1455,9 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
>>>>    			return VM_FAULT_OOM;
>>>>    	}
>>>> -	track_pfn_insert(vma, &pgprot, pfn);
>>>> +	if (pfnmap_sanitize_pgprot(pfn_t_to_pfn(pfn), PAGE_SIZE, &pgprot))
>>>> +		return VM_FAULT_FALLBACK;
>>>
>>> Would "pgtable" leak if it fails?  If it's PAGE_SIZE, IIUC it won't ever
>>> trigger, though.
>>>
>>> Maybe we could have a "void pfnmap_sanitize_pgprot_pfn(&pgprot, pfn)" to
>>> replace track_pfn_insert() and never fail?  Dropping vma ref is definitely
>>> a win already in all cases.
>>
>> It could be a simple wrapper around pfnmap_sanitize_pgprot(), yes. That's
>> certainly helpful for the single-page case.
>>
>> Regarding never failing here: we should check the whole range. We have to
>> make sure that none of the pages has a memory type / caching mode that is
>> incompatible with what we setup.
> 
> Would it happen in real world?
 > > IIUC per-vma registration needs to happen first, which checks for 
memtype
> conflicts in the first place, or reserve_pfn_range() could already have
> failed.
 > > Here it's the fault path looking up the memtype, so I would expect 
it is
> guaranteed all pfns under the same vma is following the verified (and same)
> memtype?

The whole point of track_pfn_insert() is that it is used when we *don't* 
use reserve_pfn_range()->track_pfn_remap(), no?

track_pfn_remap() would check the whole range that gets mapped, so 
track_pfn_insert() user must similarly check the whole range that gets 
mapped.

Note that even track_pfn_insert() is already pretty clear on the 
intended usage: "called when a _new_ single pfn is established"

-- 
Cheers,

David / dhildenb