Re: [PATCH v21 1/4] mm: add VM_DROPPABLE for designating always lazily freeable mappings

[David Hildenbrand <david@redhat.com>]

On 08.07.24 03:46, Jason A. Donenfeld wrote:
> Hi David,

Hi Jason,

just catching up on mails here. Most of the stuff is now clear from the 
other subthread.

[...]

>>> @@ -1851,7 +1858,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>    				 * discarded. Remap the page to page table.
>>>    				 */
>>>    				set_pte_at(mm, address, pvmw.pte, pteval);
>>> -				folio_set_swapbacked(folio);
>>> +				/* Unlike MADV_FREE mappings, VM_DROPPABLE ones
>>> +				 * never get swap backed on failure to drop. */
>>> +				if (!(vma->vm_flags & VM_DROPPABLE))
>>> +					folio_set_swapbacked(folio);
>>>    				ret = false;
>>>    				page_vma_mapped_walk_done(&pvmw);
>>>    				break;
>>
>> A note that in mm/mm-stable, "madvise_free_huge_pmd" exists to optimize
>> MADV_FREE on PMDs. I suspect we'd want to extend that one as well for
>> dropping support, but likely it would also only be a performance
>> improvmeent and not affect functonality if not handled.
> 
> That's for doing the freeing of PTEs after the fact, right? If the
> mapping was created, got filled with some data, and then sometime later
> it got MADV_FREE'd, which is the pattern people follow typically with
> MADV_FREE. If we do this as PROT_/MAP_, then that's not a case we need
> to worry about, if I understand this code correctly.

We essentially now have code to handle PMD-mapped THP: instead of first 
remapping them using PTEs to then unmap+discard via 512 PTEs (due to 
MADV_FREE being set on the folio), we can now simply unmap+discard a 
single PMD. So performance wise, this might be interesting for this 
mechanism as well (when used in combination with THP).

-- 
Cheers,

David / dhildenb