Re: [PATCH v3 11/16] mm/page-flags: reuse PG_mappedtodisk as PG_anon_exclusive for PageAnon() pages

[Vlastimil Babka <vbabka@suse.cz>]

On 4/13/22 12:28, David Hildenbrand wrote:
> On 13.04.22 10:25, Vlastimil Babka wrote:
>> On 3/29/22 18:04, David Hildenbrand wrote:
>>>   the pin will be fully reliable and stay consistent with the pages
>>>   mapped into the page table, as the bit cannot get cleared (e.g., by
>>>   fork(), KSM) while the page is pinned. For anonymous pages that
>>>   are mapped R/W, PG_anon_exclusive can be assumed to always be set
>>>   because such pages cannot possibly be shared.
>>>
>>>   The page table lock protecting the page table entry is the primary
>>>   synchronization mechanism for PG_anon_exclusive; GUP-fast that does
>>>   not take the PT lock needs special care when trying to clear the
>>>   flag.
>>>
>>>   Page table entry types and PG_anon_exclusive:
>>>   * Present: PG_anon_exclusive applies.
>>>   * Swap: the information is lost. PG_anon_exclusive was cleared.
>>>   * Migration: the entry holds this information instead.
>>>                PG_anon_exclusive was cleared.
>>>   * Device private: PG_anon_exclusive applies.
>>>   * Device exclusive: PG_anon_exclusive applies.
>>>   * HW Poison: PG_anon_exclusive is stale and not changed.
>>>
>>>   If the page may be pinned (FOLL_PIN), clearing PG_anon_exclusive is
>>>   not allowed and the flag will stick around until the page is freed
>>>   and folio->mapping is cleared.
>> 
>> Or also if it's unpinned?
> 
> I'm afraid I didn't get your question. Once the page is no longer
> pinned, we can succeed in clearing PG_anon_exclusive (just like pinning
> never happened). Does that answer your question?

Yeah it looked like a scenario that's oddly missing in that description, yet
probably obvious. Now I feel it's indeed obvious, so nevermind :)

>>> We won't be clearing PG_anon_exclusive on destructive unmapping (i.e.,
>>> zapping) of page table entries, page freeing code will handle that when
>>> also invalidate page->mapping to not indicate PageAnon() anymore.
>>> Letting information about exclusivity stick around will be an important
>>> property when adding sanity checks to unpinning code.
>>>
>>> Note that we properly clear the flag in free_pages_prepare() via
>>> PAGE_FLAGS_CHECK_AT_PREP for each individual subpage of a compound page,
>>> so there is no need to manually clear the flag.
>>>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> 
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Thanks!
> 
>> 
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -3663,6 +3663,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>  		goto out_nomap;
>>>  	}
>>>  
>>> +	/*
>>> +	 * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte
>>> +	 * must never point at an anonymous page in the swapcache that is
>>> +	 * PG_anon_exclusive. Sanity check that this holds and especially, that
>>> +	 * no filesystem set PG_mappedtodisk on a page in the swapcache. Sanity
>>> +	 * check after taking the PT lock and making sure that nobody
>>> +	 * concurrently faulted in this page and set PG_anon_exclusive.
>>> +	 */
>>> +	BUG_ON(!PageAnon(page) && PageMappedToDisk(page));
>>> +	BUG_ON(PageAnon(page) && PageAnonExclusive(page));
>>> +
>> 
>> Hmm, dunno why not VM_BUG_ON?
> 
> Getting PageAnonExclusive accidentally set by a file system would result
> in an extremely unpleasant security issue. I most surely want to catch
> something like that in any case, especially in the foreseeable future.

OK then.