Re: [PATCH v1 02/11] mm: convert track_pfn_insert() to pfnmap_sanitize_pgprot()

[Peter Xu <peterx@redhat.com>]

On Fri, Apr 25, 2025 at 09:48:50PM +0200, David Hildenbrand wrote:
> On 25.04.25 21:31, Peter Xu wrote:
> > On Fri, Apr 25, 2025 at 10:17:06AM +0200, David Hildenbrand wrote:
> > > ... by factoring it out from track_pfn_remap().
> > > 
> > > For PMDs/PUDs, actually check the full range, and trigger a fallback
> > > if we run into this "different memory types / cachemodes" scenario.
> > 
> > The current patch looks like to still pass PAGE_SIZE into the new helper at
> > all track_pfn_insert() call sites, so it seems this comment does not 100%
> > match with the code?  Or I may have misread somewhere.
> 
> No, you're right, while reshuffling the patches I forgot to add the actual
> PMD/PUD size.
> 
> > 
> > Maybe it's still easier to keep the single-pfn lookup to never fail..  more
> > below.
> > 
> 
> [...]
> 
> > >   /*
> > > @@ -1556,8 +1553,23 @@ static inline void untrack_pfn_clear(struct vm_area_struct *vma)
> > >   extern int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
> > >   			   unsigned long pfn, unsigned long addr,
> > >   			   unsigned long size);
> > > -extern void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot,
> > > -			     pfn_t pfn);
> > > +
> > > +/**
> > > + * pfnmap_sanitize_pgprot - sanitize the pgprot for a pfn range
> > 
> > Nit: s/sanitize/update|setup|.../?
> > 
> > But maybe you have good reason to use sanitize.  No strong opinions.
> 
> 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().

> 
> > 
> > > + * @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?

Thanks,

-- 
Peter Xu