| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <94fcc32f-574a-4934-b7a9-1ed8bd32a97f@kernel.org>
Date: Mon, 17 Nov 2025 18:08:57 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org
Cc: Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Baolin Wang <baolin.wang@...ux.alibaba.com>, Hugh Dickins
<hughd@...gle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Michal Hocko
<mhocko@...e.com>, Nikita Kalyazin <kalyazin@...zon.com>,
Paolo Bonzini <pbonzini@...hat.com>, Peter Xu <peterx@...hat.com>,
Sean Christopherson <seanjc@...gle.com>, Shuah Khan <shuah@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 2/4] userfaultfd, shmem: use a VMA callback to handle
UFFDIO_CONTINUE
On 17.11.25 12:46, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
>
> When userspace resolves a page fault in a shmem VMA with UFFDIO_CONTINUE
> it needs to get a folio that already exists in the pagecache backing
> that VMA.
>
> Instead of using shmem_get_folio() for that, add a get_pagecache_folio()
> method to 'struct vm_operations_struct' that will return a folio if it
> exists in the VMA's pagecache at given pgoff.
>
> Implement get_pagecache_folio() method for shmem and slightly refactor
> userfaultfd's mfill_atomic() and mfill_atomic_pte_continue() to support
> this new API.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> ---
> include/linux/mm.h | 9 +++++++
> mm/shmem.c | 20 ++++++++++++++++
> mm/userfaultfd.c | 60 ++++++++++++++++++++++++++++++----------------
> 3 files changed, 69 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d16b33bacc32..c35c1e1ac4dd 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -690,6 +690,15 @@ 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
> + /*
> + * Called by userfault to resolve UFFDIO_CONTINUE request.
> + * Should return the folio found at pgoff in the VMA's pagecache if it
> + * exists or ERR_PTR otherwise.
> + */
What are the locking +refcount rules? Without looking at the code, I
would assume we return with a folio reference held and the folio locked?
> + struct folio *(*get_pagecache_folio)(struct vm_area_struct *vma,
> + pgoff_t pgoff);
The combination of VMA + pgoff looks weird at first. Would vma + addr or
vma+vma_offset into vma be better?
But it also makes me wonder if the callback would ever even require the
VMA, or actually only vma->vm_file?
Thinking out loud, I wonder if one could just call that "get_folio" or
"get_shared_folio" (IOW, never an anon folio in a MAP_PRIVATE mapping).
> +#endif
> };
>
> #ifdef CONFIG_NUMA_BALANCING
> diff --git a/mm/shmem.c b/mm/shmem.c
> index b9081b817d28..4ac122284bff 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3260,6 +3260,20 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
> shmem_inode_unacct_blocks(inode, 1);
> return ret;
> }
> +
> +static struct folio *shmem_get_pagecache_folio(struct vm_area_struct *vma,
> + pgoff_t pgoff)
> +{
> + struct inode *inode = file_inode(vma->vm_file);
> + struct folio *folio;
> + int err;
> +
> + err = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
> + if (err)
> + return ERR_PTR(err);
> +
> + return folio;
> +}
> #endif /* CONFIG_USERFAULTFD */
>
> #ifdef CONFIG_TMPFS
> @@ -5292,6 +5306,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
> .set_policy = shmem_set_policy,
> .get_policy = shmem_get_policy,
> #endif
> +#ifdef CONFIG_USERFAULTFD
> + .get_pagecache_folio = shmem_get_pagecache_folio,
> +#endif
> };
>
> static const struct vm_operations_struct shmem_anon_vm_ops = {
> @@ -5301,6 +5318,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = {
> .set_policy = shmem_set_policy,
> .get_policy = shmem_get_policy,
> #endif
> +#ifdef CONFIG_USERFAULTFD
> + .get_pagecache_folio = shmem_get_pagecache_folio,
> +#endif
> };
>
> int shmem_init_fs_context(struct fs_context *fc)
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 8dc964389b0d..60b3183a72c0 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -382,21 +382,17 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
> unsigned long dst_addr,
> uffd_flags_t flags)
> {
> - 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);
> + folio = dst_vma->vm_ops->get_pagecache_folio(dst_vma, pgoff);
> /* Our caller expects us to return -EFAULT if we failed to find folio */
> - if (ret == -ENOENT)
> - ret = -EFAULT;
> - if (ret)
> - goto out;
> - if (!folio) {
> - ret = -EFAULT;
> - goto out;
> + if (IS_ERR_OR_NULL(folio)) {
> + if (PTR_ERR(folio) == -ENOENT || !folio)
> + return -EFAULT;
> + return PTR_ERR(folio);
> }
>
> page = folio_file_page(folio, pgoff);
> @@ -411,13 +407,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
> goto out_release;
>
> folio_unlock(folio);
> - ret = 0;
> -out:
> - return ret;
> + return 0;
> +
> out_release:
> folio_unlock(folio);
> folio_put(folio);
> - goto out;
> + return ret;
> }
>
> /* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
> @@ -694,6 +689,22 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
> return err;
> }
>
> +static __always_inline bool vma_can_mfill_atomic(struct vm_area_struct *vma,
> + uffd_flags_t flags)
> +{
> + if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
> + if (vma->vm_ops && vma->vm_ops->get_pagecache_folio)
> + return true;
> + else
> + return false;
Probably easier to read is
return vma->vm_ops && vma->vm_ops->get_pagecache_folio;
> + }
> +
> + if (vma_is_anonymous(vma) || vma_is_shmem(vma))
> + return true;
> +
> + return false;
Could also be simplified to:
return vma_is_anonymous(vma) || vma_is_shmem(vma);
--
Cheers
David
Powered by blists - more mailing lists