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

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jul 07, 2023 at 08:54:33PM +0200, David Hildenbrand wrote:
> 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 ...

Oh, I agree, there are always going to be circumstances where we realise
we've made a bad decision and can't (easily) undo it.  Unless we have a
per-page pincount, and I Would Rather Not Do That.  But we should _try_
to do that because it's the right model -- that's what I meant by "Tell
me why I'm wrong"; what scenarios do we have where a user temporarilly
mlocks (or mprotects or ...) a range of memory, but wants that memory
to be aged in the LRU exactly the same way as the adjacent memory that
wasn't mprotected?

GUP-pinning is different, and I don't think GUP-pinning should split
a folio.  That's a temporary use (not FOLL_LONGTERM), eg, we're doing
tcp zero-copy or it's the source/target of O_DIRECT.  That's not an
instruction that this memory is different from its neighbours.

Maybe we end up deciding to split folios on GUP-pin.  That would be
regrettable.