Re: [PATCH v2 1/4] mm: Introduce vm_uffd_ops API

[Mike Rapoport <rppt@kernel.org>]

On Thu, Sep 18, 2025 at 12:47:41PM -0400, Liam R. Howlett wrote:
> * Mike Rapoport <rppt@kernel.org> [250918 04:37]:
> > On Wed, Sep 17, 2025 at 12:53:05PM -0400, Liam R. Howlett wrote:
> > >
> > > I disagree, the filesystem vma_ops->fault() is not a config option like
> > > this one.  So we are on a path to enable uffd by default, and it really
> > > needs work beyond this series.  Setting up a list head and passing in
> > > through every call stack is far from idea.
> > 
> > I don't follow you here. How addition of uffd callbacks guarded by a config
> > option to vma_ops leads to enabling uffd by by default?
> 
> Any new memory type that uses the above interface now needs uffd
> enabled, anyone planning to use guest_memfd needs it enabled, anyone
> able to get a module using this interface needs it enabled (by whoever
> gives them the kernel they use).  Kernel provides now need to enable
> UFFD - which is different than the example provided.

My understanding of Peter's suggestion is that *if* uffd is enabled memory
type *may* implement the API, whatever API we'll come up with.
  
> > Nevertheless, let's step back for a second and instead focus on the problem
> > these patches are trying to solve, which is to allow guest_memfd implement
> > UFFD_CONTINUE (or minor fault in other terminology). 
> 
> Well, this is about modularizing memory types, but the first user is
> supposed to be the guest-memfd support.
> 
> > 
> > This means uffd should be able to map a folio that's already in
> > guest_memfd page cache to the faulted address. Obviously, the page table
> > update happens in uffd. But it still has to find what to map and we need
> > some way to let guest_memfd tell that to uffd.
> > 
> > So we need a hook somewhere that will return a folio matching pgoff in
> > vma->file->inode.
> > 
> > Do you see a way to implement it otherwise?
> 
> I must be missing something.
> 
> UFFDIO_CONTINUE currently enters through an ioctl that calls
> userfaultfd_continue() -> mfill_atomic_continue()... mfill_atomic() gets
> and uses the folio to actually do the work.  Right now, we don't hand
> out the folio, so what is different here?

The ioctl() is the mean of userspace to resolve a page fault and
mfill_atomic() needs something similar to ->fault() to actually get the
folio. And in case of shmem and guest_memfd the folio lives in the page
cache.
 
> I am under the impression that we don't need to return the folio, but
> may need to do work on it.  That is, we can give the mm side what it
> needs to call the related memory type functions to service the request.
> 
> For example, one could pass in the inode, pgoff, and memory type and the
> mm code could then call the fault handler for that memory type?

How calling the fault handler differs conceptually from calling
uffd_get_folio?
If you take a look at UFFD_CONTINUE for shmem, this is pretty much what's
happening. uffd side finds inode and pgoff and calls to a shmem_get_folio()
that's very much similar to shmem->fault().

-- 
Sincerely yours,
Mike.