Re: [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk_start()

[David Hildenbrand <david@redhat.com>]

On 09.08.24 18:54, Peter Xu wrote:
> On Fri, Aug 09, 2024 at 06:20:06PM +0200, David Hildenbrand wrote:
>> On 09.08.24 18:08, Peter Xu wrote:
>>> Pfnmaps can always be identified with special bits in the ptes/pmds/puds.
>>> However that's unnecessary if the vma is stable, and when it's mapped under
>>> VM_PFNMAP | VM_IO.
>>>
>>> Instead of adding similar checks in all the levels for huge pfnmaps, let
>>> folio_walk_start() fail even earlier for these mappings.  It's also
>>> something gup-slow already does, so make them match.
>>>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>>    mm/pagewalk.c | 5 +++++
>>>    1 file changed, 5 insertions(+)
>>>
>>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>>> index cd79fb3b89e5..fd3965efe773 100644
>>> --- a/mm/pagewalk.c
>>> +++ b/mm/pagewalk.c
>>> @@ -727,6 +727,11 @@ struct folio *folio_walk_start(struct folio_walk *fw,
>>>    	p4d_t *p4dp;
>>>    	mmap_assert_locked(vma->vm_mm);
>>> +
>>> +	/* It has no folio backing the mappings at all.. */
>>> +	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
>>> +		return NULL;
>>> +
>>
>> That is in general not what we want, and we still have some places that
>> wrongly hard-code that behavior.
>>
>> In a MAP_PRIVATE mapping you might have anon pages that we can happily walk.
>>
>> vm_normal_page() / vm_normal_page_pmd() [and as commented as a TODO,
>> vm_normal_page_pud()] should be able to identify PFN maps and reject them,
>> no?
> 
> Yep, I think we can also rely on special bit.
> 
> When I was working on this whole series I must confess I am already
> confused on the real users of MAP_PRIVATE pfnmaps.  E.g. we probably don't
> need either PFNMAP for either mprotect/fork/... at least for our use case,
> then VM_PRIVATE is even one step further.

Yes, it's rather a corner case indeed.
> 
> Here I chose to follow gup-slow, and I suppose you meant that's also wrong?

I assume just nobody really noticed, just like nobody noticed that 
walk_page_test() skips VM_PFNMAP (but not VM_IO :) ).

Your process memory stats will likely miss anon folios on COW PFNMAP 
mappings ... in the rare cases where they exist (e.g., mmap() of /dev/mem).

> If so, would it make sense we keep them aligned as of now, and change them
> altogether?  Or do you think we should just rely on the special bits?

GUP already refuses to work on a lot of other stuff, so likely not a 
good use of time unless somebody complains.

But yes, long-term we should make all code either respect that it could 
happen (and bury less awkward checks in page table walkers) or rip 
support for MAP_PRIVATE PFNMAP out completely.

> 
> And, just curious: is there any use case you're aware of that can benefit
> from caring PRIVATE pfnmaps yet so far, especially in this path?

In general MAP_PRIVATE pfnmaps is not really useful on things like MMIO.

There was a discussion (in VM_PAT) some time ago whether we could remove 
MAP_PRIVATE PFNMAPs completely [1]. At least some users still use COW 
mappings on /dev/mem, although not many (and they might not actually 
write to these areas).

I'm happy if someone wants to try ripping that out, I'm not brave enough :)

[1] 
https://lkml.kernel.org/r/1f2a8ed4-aaff-4be7-b3b6-63d2841a2908@redhat.com

> 
> As far as I read, none of folio_walk_start() users so far should even
> stumble on top of a pfnmap, share or private.  But that's a fairly quick
> glimps only.

do_pages_stat()->do_pages_stat_array() should be able to trigger it, if 
you pass "nodes=NULL" to move_pages().

Maybe s390x could be tricked into it, but likely as you say, most code 
shouldn't trigger it. The function itself should be handling it 
correctly as of today, though.

-- 
Cheers,

David / dhildenb