Re: [RFC PATCH 0/3] support large folio for mlock

[David Hildenbrand <david@redhat.com>]

On 07.07.23 19:26, Matthew Wilcox wrote:
> On Sat, Jul 08, 2023 at 12:52:18AM +0800, Yin Fengwei wrote:
>> This series identified the large folio for mlock to two types:
>>    - The large folio is in VM_LOCKED VMA range
>>    - The large folio cross VM_LOCKED VMA boundary
> 
> This is somewhere that I think our fixation on MUST USE PMD ENTRIES
> has led us astray.  Today when the arguments to mlock() cross a folio
> boundary, we split the PMD entry but leave the folio intact.  That means
> that we continue to manage the folio as a single entry on the LRU list.
> But userspace may have no idea that we're doing this.  It may have made
> several calls to mmap() 256kB at once, they've all been coalesced into
> a single VMA and khugepaged has come along behind its back and created
> a 2MB THP.  Now userspace calls mlock() and instead of treating that as
> a hint that oops, maybe we shouldn't've done that, we do our utmost to
> preserve the 2MB folio.
> 
> I think this whole approach needs rethinking.  IMO, anonymous folios
> should not cross VMA boundaries.  Tell me why I'm wrong.

I think we touched upon that a couple of times already, and the main 
issue is that while it sounds nice in theory, it's impossible in practice.

THP are supposed to be transparent, that is, we should not let arbitrary 
operations fail.

But nothing stops user space from

(a) mmap'ing a 2 MiB region
(b) GUP-pinning the whole range
(c) GUP-pinning the first half
(d) unpinning the whole range from (a)
(e) munmap'ing the second half


And that's just one out of many examples I can think of, not even 
considering temporary/speculative references that can prevent a split at 
random points in time -- especially when splitting a VMA.

Sure, any time we PTE-map a THP we might just say "let's put that on the 
deferred split queue" and cross fingers that we can eventually split it 
later. (I was recently thinking about that in the context of the 
mapcount ...)

It's all a big mess ...

-- 
Cheers,

David / dhildenb