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Message-ID: <aRA-se6hJPzHD3mA@kernel.org>
Date: Sun, 9 Nov 2025 09:11:45 +0200
From: Mike Rapoport <rppt@...nel.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
"David Hildenbrand (Red Hat)" <david@...nel.org>,
Peter Xu <peterx@...hat.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Muchun Song <muchun.song@...ux.dev>,
Nikita Kalyazin <kalyazin@...zon.com>,
Vlastimil Babka <vbabka@...e.cz>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
James Houghton <jthoughton@...gle.com>,
Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...e.com>,
Ujwal Kundur <ujwal.kundur@...il.com>,
Oscar Salvador <osalvador@...e.de>,
Suren Baghdasaryan <surenb@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>, conduct@...nel.org
Subject: Re: [PATCH v4 0/4] mm/userfaultfd: modulize memory types
Hi Liam,
On Thu, Nov 06, 2025 at 11:32:46AM -0500, Liam R. Howlett wrote:
> * Mike Rapoport <rppt@...nel.org> [251104 02:22]:
> > On Mon, Nov 03, 2025 at 10:27:05PM +0100, David Hildenbrand (Red Hat) wrote:
> > >
> > > And maybe that's the main problem here: Liam talks about general uffd
> > > cleanups while you are focused on supporting guest_memfd minor mode "as
> > > simple as possible" (as you write below).
> >
> > Hijacking for the technical part for a moment ;-)
> >
> > It seems that "as simple as possible" can even avoid data members in struct
> > vm_uffd_ops, e.g something along these lines:
>
> I like this because it removes the flag.
>
> If we don't want to return the folio, we could modify the
> mfill_atomic_pte_continue() to __mfill_atomic_pte_continue() which takes
> a function pointer and have the callers pass a different get_folio() by
> memory type. Each memory type (anon, shmem, and guest_memfd) would have
> a small stub that would be set in the vm_ops.
I'm not sure I follow you here.
What do you mean by "don't want to return the folio"?
Isn't ->minor_get_folio() is already a different get_folio() by memory
type?
> It also looks similar to vma_get_uffd_ops() in 1fa9377e57eb1
> ("mm/userfaultfd: Introduce userfaultfd ops and use it for destination
> validation") [1]. But I always returned a uffd ops, which passes all
> uffd testing. When would your NULL uffd ops be hit? That is, when
> would uffd_ops not be set and not be anon?
The patch is a prototype. Quite possibly you are right and there's no need
to return NULL there.
> [1]. https://git.infradead.org/?p=users/jedix/linux-maple.git;a=blobdiff;f=mm/userfaultfd.c;h=e2570e72242e5a350508f785119c5dee4d8176c1;hp=e8341a45e7e8d239c64f460afeb5b2b8b29ed853;hb=1fa9377e57eb16d7fa579ea7f8eb832164d209ac;hpb=2166e91882eb195677717ac2f8fbfc58171196ce
>
> Thanks,
> Liam
>
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index d16b33bacc32..840986780cb5 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -605,6 +605,8 @@ struct vm_fault {
> > */
> > };
> >
> > +struct vm_uffd_ops;
> > +
> > /*
> > * These are the virtual MM functions - opening of an area, closing and
> > * unmapping it (needed to keep files on disk up-to-date etc), pointer
> > @@ -690,6 +692,9 @@ struct vm_operations_struct {
> > struct page *(*find_normal_page)(struct vm_area_struct *vma,
> > unsigned long addr);
> > #endif /* CONFIG_FIND_NORMAL_PAGE */
> > +#ifdef CONFIG_USERFAULTFD
> > + const struct vm_uffd_ops *uffd_ops;
> > +#endif
> > };
> >
> > #ifdef CONFIG_NUMA_BALANCING
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index c0e716aec26a..aac7ac616636 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -111,6 +111,11 @@ static inline uffd_flags_t uffd_flags_set_mode(uffd_flags_t flags, enum mfill_at
> > /* Flags controlling behavior. These behavior changes are mode-independent. */
> > #define MFILL_ATOMIC_WP MFILL_ATOMIC_FLAG(0)
> >
> > +struct vm_uffd_ops {
> > + int (*minor_get_folio)(struct inode *inode, pgoff_t pgoff,
> > + struct folio **folio);
> > +};
> > +
> > extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
> > struct vm_area_struct *dst_vma,
> > unsigned long dst_addr, struct page *page,
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index b9081b817d28..b4318ad3bdf9 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -3260,6 +3260,17 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
> > shmem_inode_unacct_blocks(inode, 1);
> > return ret;
> > }
> > +
> > +static int shmem_uffd_minor_get_folio(struct inode *inode, pgoff_t pgoff,
> > + struct folio **folio)
> > +{
> > + return shmem_get_folio(inode, pgoff, 0, folio, SGP_NOALLOC);
> > +}
> > +
> > +static const struct vm_uffd_ops shmem_uffd_ops = {
> > + .minor_get_folio = shmem_uffd_minor_get_folio,
> > +};
> > +
> > #endif /* CONFIG_USERFAULTFD */
> >
> > #ifdef CONFIG_TMPFS
> > @@ -5292,6 +5303,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
> > .set_policy = shmem_set_policy,
> > .get_policy = shmem_get_policy,
> > #endif
> > +#ifdef CONFIG_USERFAULTFD
> > + .uffd_ops = &shmem_uffd_ops,
> > +#endif
> > };
> >
> > static const struct vm_operations_struct shmem_anon_vm_ops = {
> > @@ -5301,6 +5315,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = {
> > .set_policy = shmem_set_policy,
> > .get_policy = shmem_get_policy,
> > #endif
> > +#ifdef CONFIG_USERFAULTFD
> > + .uffd_ops = &shmem_uffd_ops,
> > +#endif
> > };
> >
> > int shmem_init_fs_context(struct fs_context *fc)
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index af61b95c89e4..6b30a8f39f4d 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -20,6 +20,20 @@
> > #include "internal.h"
> > #include "swap.h"
> >
> > +static const struct vm_uffd_ops anon_uffd_ops = {
> > +};
> > +
> > +static inline const struct vm_uffd_ops *vma_get_uffd_ops(struct vm_area_struct *vma)
> > +{
> > + if (vma->vm_ops && vma->vm_ops->uffd_ops)
> > + return vma->vm_ops->uffd_ops;
> > +
> > + if (vma_is_anonymous(vma))
> > + return &anon_uffd_ops;
> > +
> > + return NULL;
> > +}
> > +
> > static __always_inline
> > bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
> > {
> > @@ -382,13 +396,14 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
> > unsigned long dst_addr,
> > uffd_flags_t flags)
> > {
> > + const struct vm_uffd_ops *uffd_ops = vma_get_uffd_ops(dst_vma);
> > struct inode *inode = file_inode(dst_vma->vm_file);
> > pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
> > struct folio *folio;
> > struct page *page;
> > int ret;
> >
> > - ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
> > + ret = uffd_ops->minor_get_folio(inode, pgoff, &folio);
> > /* Our caller expects us to return -EFAULT if we failed to find folio */
> > if (ret == -ENOENT)
> > ret = -EFAULT;
> > @@ -707,6 +722,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> > unsigned long src_addr, dst_addr;
> > long copied;
> > struct folio *folio;
> > + const struct vm_uffd_ops *uffd_ops;
> >
> > /*
> > * Sanitize the command parameters:
> > @@ -766,10 +782,11 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> > return mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
> > src_start, len, flags);
> >
> > - if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
> > + uffd_ops = vma_get_uffd_ops(dst_vma);
> > + if (!uffd_ops)
> > goto out_unlock;
> > - if (!vma_is_shmem(dst_vma) &&
> > - uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> > + if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE) &&
> > + !uffd_ops->minor_get_folio)
> > goto out_unlock;
> >
> > while (src_addr < src_start + len) {
> >
> > --
> > Sincerely yours,
> > Mike.
--
Sincerely yours,
Mike.
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