Re: [PATCH v2 01/16] mm: correctly handle UFFD PTE markers

[Lorenzo Stoakes]

On Mon, Nov 10, 2025 at 03:44:43PM +0200, Mike Rapoport wrote:
> On Mon, Nov 10, 2025 at 01:01:36PM +0000, Lorenzo Stoakes wrote:
> > On Mon, Nov 10, 2025 at 01:17:37PM +0200, Mike Rapoport wrote:
> > > On Sat, Nov 08, 2025 at 05:08:15PM +0000, Lorenzo Stoakes wrote:
> > > > PTE markers were previously only concerned with UFFD-specific logic - that
> > > > is, PTE entries with the UFFD WP marker set or those marked via
> > > > UFFDIO_POISON.
> > > >
> > > > However since the introduction of guard markers in commit
> > > >  7c53dfbdb024 ("mm: add PTE_MARKER_GUARD PTE marker"), this has no longer
> > > >  been the case.
> > > >
> > > > Issues have been avoided as guard regions are not permitted in conjunction
> > > > with UFFD, but it still leaves very confusing logic in place, most notably
> > > > the misleading and poorly named pte_none_mostly() and
> > > > huge_pte_none_mostly().
> > > >
> > > > This predicate returns true for PTE entries that ought to be treated as
> > > > none, but only in certain circumstances, and on the assumption we are
> > > > dealing with H/W poison markers or UFFD WP markers.
> > > >
> > > > This patch removes these functions and makes each invocation of these
> > > > functions instead explicitly check what it needs to check.
> > > >
> > > > As part of this effort it introduces is_uffd_pte_marker() to explicitly
> > > > determine if a marker in fact is used as part of UFFD or not.
> > > >
> > > > In the HMM logic we note that the only time we would need to check for a
> > > > fault is in the case of a UFFD WP marker, otherwise we simply encounter a
> > > > fault error (VM_FAULT_HWPOISON for H/W poisoned marker, VM_FAULT_SIGSEGV
> > > > for a guard marker), so only check for the UFFD WP case.
> > > >
> > > > While we're here we also refactor code to make it easier to understand.
> > > >
> > > > Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> > > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > > > ---
> > > >  fs/userfaultfd.c              | 83 +++++++++++++++++++----------------
> > > >  include/asm-generic/hugetlb.h |  8 ----
> > > >  include/linux/swapops.h       | 18 --------
> > > >  include/linux/userfaultfd_k.h | 21 +++++++++
> > > >  mm/hmm.c                      |  2 +-
> > > >  mm/hugetlb.c                  | 47 ++++++++++----------
> > > >  mm/mincore.c                  | 17 +++++--
> > > >  mm/userfaultfd.c              | 27 +++++++-----
> > > >  8 files changed, 123 insertions(+), 100 deletions(-)
> > > >
> > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > > index 54c6cc7fe9c6..04c66b5001d5 100644
> > > > --- a/fs/userfaultfd.c
> > > > +++ b/fs/userfaultfd.c
> > > > @@ -233,40 +233,46 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > > >  {
> > > >  	struct vm_area_struct *vma = vmf->vma;
> > > >  	pte_t *ptep, pte;
> > > > -	bool ret = true;
> > > >
> > > >  	assert_fault_locked(vmf);
> > > >
> > > >  	ptep = hugetlb_walk(vma, vmf->address, vma_mmu_pagesize(vma));
> > > >  	if (!ptep)
> > > > -		goto out;
> > > > +		return true;
> > > >
> > > > -	ret = false;
> > > >  	pte = huge_ptep_get(vma->vm_mm, vmf->address, ptep);
> > > >
> > > >  	/*
> > > >  	 * Lockless access: we're in a wait_event so it's ok if it
> > > > -	 * changes under us.  PTE markers should be handled the same as none
> > > > -	 * ptes here.
> > > > +	 * changes under us.
> > > >  	 */
> > > > -	if (huge_pte_none_mostly(pte))
> > > > -		ret = true;
> > > > +
> > > > +	/* If missing entry, wait for handler. */
> > >
> > > It's actually #PF handler that waits ;-)
> >
> > Think I meant uffd userland 'handler' as in handle_userfault(). But this is not
> > clear obviously.
> >
> > >
> > > When userfaultfd_(huge_)must_wait() return true, it means that process that
> > > caused a fault should wait until userspace resolves the fault and return
> > > false means that it's ok to retry the #PF.
> >
> > Yup.
> >
> > >
> > > So the comment here should probably read as
> > >
> > > 	/* entry is still missing, wait for userspace to resolve the fault */
> > >
> >
> > Will update to make clearer thanks.
> >
> > >
> > > > +	if (huge_pte_none(pte))
> > > > +		return true;
> > > > +	/* UFFD PTE markers require handling. */
> > > > +	if (is_uffd_pte_marker(pte))
> > > > +		return true;
> > > > +	/* If VMA has UFFD WP faults enabled and WP fault, wait for handler. */
> > > >  	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
> > > > -		ret = true;
> > > > -out:
> > > > -	return ret;
> > > > +		return true;
> > > > +
> > > > +	/* Otherwise, if entry isn't present, let fault handler deal with it. */
> > >
> > > Entry is actually present here, e.g because there is a thread that called
> > > UFFDIO_COPY in parallel with the fault, so no need to stuck the faulting
> > > process.
> >
> > Well it might not be? Could be a swap entry, migration entry, etc. unless I'm
> > missing cases? Point of comment was 'ok if non-present in a way that doesn't
> > require a userfaultfd userland handler the fault handler will deal'
> >
> > But anyway agree this isn't clear, probably better to just say 'otherwise no
> > need for userland uffd handler to do anything here' or similar.
>
> It's not that userspace does not need to do anything, it's just that pte is
> good enough for the faulting thread to retry the page fault without waiting
> for userspace to resolve the fault.

OK I will clarify that in the comment.

>
> > Cheers, Lorenzo
>
> --
> Sincerely yours,
> Mike.