Re: [PATCH v3 kvm/queue 03/16] mm/memfd: Introduce MEMFD_OPS

From: Robert Hoo
Date: Thu Dec 23 2021 - 22:53:28 EST

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On Thu, 2021-12-23 at 20:29 +0800, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> The patch introduces new MEMFD_OPS facility around file created by
> memfd_create() to allow a third kernel component to make use of
> memory
> bookmarked in a memfd and gets notifier when the memory in the file
> is allocated/invalidated. It will be used for KVM to use memfd file
> descriptor as the guest memory backend and KVM will use MEMFD_OPS to
> interact with memfd subsystem. In the future there might be other
> consumers (e.g. VFIO with encrypted device memory).
>
> It consists two set of callbacks:
> - memfd_falloc_notifier: callbacks which provided by KVM and called
> by memfd when memory gets allocated/invalidated through
> fallocate()
> ioctl.
> - memfd_pfn_ops: callbacks which provided by memfd and called by
> KVM
> to request memory page from memfd.
>
> Locking is needed for above callbacks to prevent race condition.
> - get_owner/put_owner is used to ensure the owner is still alive in
> the invalidate_page_range/fallocate callback handlers using a
> reference mechanism.
> - page is locked between get_lock_pfn/put_unlock_pfn to ensure pfn
> is
> still valid when it's used (e.g. when KVM page fault handler uses
> it to establish the mapping in the secondary MMU page tables).
>
> Userspace is in charge of guest memory lifecycle: it can allocate the
> memory with fallocate() or punch hole to free memory from the guest.
>
> The file descriptor passed down to KVM as guest memory backend. KVM
> registers itself as the owner of the memfd via
> memfd_register_falloc_notifier() and provides memfd_falloc_notifier
> callbacks that need to be called on fallocate() and punching hole.
>
> memfd_register_falloc_notifier() returns memfd_pfn_ops callbacks that
> need to be used for requesting a new page from KVM.
>
> At this time only shmem is supported.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
> include/linux/memfd.h | 22 ++++++
> include/linux/shmem_fs.h | 16 ++++
> mm/Kconfig | 4 +
> mm/memfd.c | 21 ++++++
> mm/shmem.c | 158
> +++++++++++++++++++++++++++++++++++++++
> 5 files changed, 221 insertions(+)
>
> diff --git a/include/linux/memfd.h b/include/linux/memfd.h
> index 4f1600413f91..0007073b53dc 100644
> --- a/include/linux/memfd.h
> +++ b/include/linux/memfd.h
> @@ -13,4 +13,26 @@ static inline long memfd_fcntl(struct file *f,
> unsigned int c, unsigned long a)
> }
> #endif
>
> +#ifdef CONFIG_MEMFD_OPS
> +struct memfd_falloc_notifier {
> + void (*invalidate_page_range)(struct inode *inode, void *owner,
> + pgoff_t start, pgoff_t end);
> + void (*fallocate)(struct inode *inode, void *owner,
> + pgoff_t start, pgoff_t end);
> + bool (*get_owner)(void *owner);
> + void (*put_owner)(void *owner);
> +};
> +
> +struct memfd_pfn_ops {
> + long (*get_lock_pfn)(struct inode *inode, pgoff_t offset, int
> *order);
> + void (*put_unlock_pfn)(unsigned long pfn);
> +
> +};
> +
> +extern int memfd_register_falloc_notifier(struct inode *inode, void
> *owner,
> + const struct memfd_falloc_notifier
> *notifier,
> + const struct memfd_pfn_ops **pfn_ops);
> +extern void memfd_unregister_falloc_notifier(struct inode *inode);
> +#endif
> +
> #endif /* __LINUX_MEMFD_H */
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 166158b6e917..503adc63728c 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -12,6 +12,11 @@
>
> /* inode in-kernel data */
>
> +#ifdef CONFIG_MEMFD_OPS
> +struct memfd_falloc_notifier;
> +struct memfd_pfn_ops;
> +#endif
> +
> struct shmem_inode_info {
> spinlock_t lock;
> unsigned int seals; /* shmem seals */
> @@ -24,6 +29,10 @@ struct shmem_inode_info {
> struct shared_policy policy; /* NUMA memory alloc
> policy */
> struct simple_xattrs xattrs; /* list of xattrs */
> atomic_t stop_eviction; /* hold when working on inode
> */
> +#ifdef CONFIG_MEMFD_OPS
> + void *owner;
> + const struct memfd_falloc_notifier *falloc_notifier;
> +#endif
> struct inode vfs_inode;
> };
>
> @@ -96,6 +105,13 @@ extern unsigned long shmem_swap_usage(struct
> vm_area_struct *vma);
> extern unsigned long shmem_partial_swap_usage(struct address_space
> *mapping,
> pgoff_t start, pgoff_t
> end);
>
> +#ifdef CONFIG_MEMFD_OPS
> +extern int shmem_register_falloc_notifier(struct inode *inode, void
> *owner,
> + const struct memfd_falloc_notifier
> *notifier,
> + const struct memfd_pfn_ops **pfn_ops);
> +extern void shmem_unregister_falloc_notifier(struct inode *inode);
> +#endif
> +
> /* Flag allocation requirements to shmem_getpage */
> enum sgp_type {
> SGP_READ, /* don't exceed i_size, don't allocate page */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 28edafc820ad..9989904d1b56 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -900,6 +900,10 @@ config IO_MAPPING
> config SECRETMEM
> def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED
>
> +config MEMFD_OPS
> + bool
> + depends on MEMFD_CREATE
> +
> source "mm/damon/Kconfig"
>
> endmenu
> diff --git a/mm/memfd.c b/mm/memfd.c
> index c898a007fb76..41861870fc21 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -130,6 +130,27 @@ static unsigned int *memfd_file_seals_ptr(struct
> file *file)
> return NULL;
> }
>
> +#ifdef CONFIG_MEMFD_OPS
> +int memfd_register_falloc_notifier(struct inode *inode, void *owner,
> + const struct memfd_falloc_notifier
> *notifier,
> + const struct memfd_pfn_ops
> **pfn_ops)
> +{
> + if (shmem_mapping(inode->i_mapping))
> + return shmem_register_falloc_notifier(inode, owner,
> + notifier,
> pfn_ops);
> +
> + return -EINVAL;
> +}
> +EXPORT_SYMBOL_GPL(memfd_register_falloc_notifier);
> +
> +void memfd_unregister_falloc_notifier(struct inode *inode)
> +{
> + if (shmem_mapping(inode->i_mapping))
> + shmem_unregister_falloc_notifier(inode);
> +}
> +EXPORT_SYMBOL_GPL(memfd_unregister_falloc_notifier);
> +#endif
> +
> #define F_ALL_SEALS (F_SEAL_SEAL | \
> F_SEAL_SHRINK | \
> F_SEAL_GROW | \
> diff --git a/mm/shmem.c b/mm/shmem.c
> index faa7e9b1b9bc..4d8a75c4d037 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -78,6 +78,7 @@ static struct vfsmount *shm_mnt;
> #include <linux/userfaultfd_k.h>
> #include <linux/rmap.h>
> #include <linux/uuid.h>
> +#include <linux/memfd.h>
>
> #include <linux/uaccess.h>
>
> @@ -906,6 +907,68 @@ static bool shmem_punch_compound(struct page
> *page, pgoff_t start, pgoff_t end)
> return split_huge_page(page) >= 0;
> }
>
> +static void notify_fallocate(struct inode *inode, pgoff_t start,
> pgoff_t end)
> +{
> +#ifdef CONFIG_MEMFD_OPS
> + struct shmem_inode_info *info = SHMEM_I(inode);
> + const struct memfd_falloc_notifier *notifier;
> + void *owner;
> + bool ret;
> +
> + if (!info->falloc_notifier)
> + return;
> +
> + spin_lock(&info->lock);
> + notifier = info->falloc_notifier;
> + if (!notifier) {
> + spin_unlock(&info->lock);
> + return;
> + }
> +
> + owner = info->owner;
> + ret = notifier->get_owner(owner);
> + spin_unlock(&info->lock);
> + if (!ret)
> + return;
> +
> + notifier->fallocate(inode, owner, start, end);

I see notifier->fallocate(), i.e. memfd_fallocate(), discards
kvm_memfd_fallocate_range()'s return value. Should it be checked?

> + notifier->put_owner(owner);
> +#endif
> +}
> +
> +static void notify_invalidate_page(struct inode *inode, struct page
> *page,
> + pgoff_t start, pgoff_t end)
> +{
> +#ifdef CONFIG_MEMFD_OPS
> + struct shmem_inode_info *info = SHMEM_I(inode);
> + const struct memfd_falloc_notifier *notifier;
> + void *owner;
> + bool ret;
> +
> + if (!info->falloc_notifier)
> + return;
> +
> + spin_lock(&info->lock);
> + notifier = info->falloc_notifier;
> + if (!notifier) {
> + spin_unlock(&info->lock);
> + return;
> + }
> +
> + owner = info->owner;
> + ret = notifier->get_owner(owner);
> + spin_unlock(&info->lock);
> + if (!ret)
> + return;
> +
> + start = max(start, page->index);
> + end = min(end, page->index + thp_nr_pages(page));
> +
> + notifier->invalidate_page_range(inode, owner, start, end);
> + notifier->put_owner(owner);
> +#endif
> +}
> +
> /*
> * Remove range of pages and swap entries from page cache, and free
> them.
> * If !unfalloc, truncate or punch hole; if unfalloc, undo failed
> fallocate.
> @@ -949,6 +1012,8 @@ static void shmem_undo_range(struct inode
> *inode, loff_t lstart, loff_t lend,
> }
> index += thp_nr_pages(page) - 1;
>
> + notify_invalidate_page(inode, page, start,
> end);
> +
> if (!unfalloc || !PageUptodate(page))
> truncate_inode_page(mapping, page);
> unlock_page(page);
> @@ -1025,6 +1090,9 @@ static void shmem_undo_range(struct inode
> *inode, loff_t lstart, loff_t lend,
> index--;
> break;
> }
> +
> + notify_invalidate_page(inode, page,
> start, end);
> +
> VM_BUG_ON_PAGE(PageWriteback(page),
> page);
> if (shmem_punch_compound(page, start,
> end))
> truncate_inode_page(mapping,
> page);
> @@ -2815,6 +2883,7 @@ static long shmem_fallocate(struct file *file,
> int mode, loff_t offset,
> if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode-
> >i_size)
> i_size_write(inode, offset + len);
> inode->i_ctime = current_time(inode);
> + notify_fallocate(inode, start, end);
> undone:
> spin_lock(&inode->i_lock);
> inode->i_private = NULL;
> @@ -3784,6 +3853,20 @@ static void shmem_destroy_inodecache(void)
> kmem_cache_destroy(shmem_inode_cachep);
> }
>
> +#ifdef CONFIG_MIGRATION
> +int shmem_migrate_page(struct address_space *mapping, struct page
> *newpage,
> + struct page *page, enum migrate_mode mode)
> +{
> +#ifdef CONFIG_MEMFD_OPS
> + struct inode *inode = mapping->host;
> +
> + if (SHMEM_I(inode)->owner)
> + return -EOPNOTSUPP;
> +#endif
> + return migrate_page(mapping, newpage, page, mode);
> +}
> +#endif
> +
> const struct address_space_operations shmem_aops = {
> .writepage = shmem_writepage,
> .set_page_dirty = __set_page_dirty_no_writeback,
> @@ -3798,6 +3881,81 @@ const struct address_space_operations
> shmem_aops = {
> };
> EXPORT_SYMBOL(shmem_aops);
>
> +#ifdef CONFIG_MEMFD_OPS
> +static long shmem_get_lock_pfn(struct inode *inode, pgoff_t offset,
> int *order)
> +{
> + struct page *page;
> + int ret;
> +
> + ret = shmem_getpage(inode, offset, &page, SGP_NOALLOC);
> + if (ret)
> + return ret;
> +
> + *order = thp_order(compound_head(page));
> +
> + return page_to_pfn(page);
> +}
> +
> +static void shmem_put_unlock_pfn(unsigned long pfn)
> +{
> + struct page *page = pfn_to_page(pfn);
> +
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> +
> + set_page_dirty(page);
> + unlock_page(page);
> + put_page(page);
> +}
> +
> +static const struct memfd_pfn_ops shmem_pfn_ops = {
> + .get_lock_pfn = shmem_get_lock_pfn,
> + .put_unlock_pfn = shmem_put_unlock_pfn,
> +};
> +
> +int shmem_register_falloc_notifier(struct inode *inode, void *owner,
> + const struct memfd_falloc_notifier
> *notifier,
> + const struct memfd_pfn_ops **pfn_ops)
> +{
> + gfp_t gfp;
> + struct shmem_inode_info *info = SHMEM_I(inode);
> +
> + if (!inode || !owner || !notifier || !pfn_ops ||
> + !notifier->invalidate_page_range ||
> + !notifier->fallocate ||
> + !notifier->get_owner ||
> + !notifier->put_owner)
> + return -EINVAL;
> +
> + spin_lock(&info->lock);
> + if (info->owner && info->owner != owner) {
> + spin_unlock(&info->lock);
> + return -EPERM;
> + }
> +
> + info->owner = owner;
> + info->falloc_notifier = notifier;
> + spin_unlock(&info->lock);
> +
> + gfp = mapping_gfp_mask(inode->i_mapping);
> + gfp &= ~__GFP_MOVABLE;
> + mapping_set_gfp_mask(inode->i_mapping, gfp);
> + mapping_set_unevictable(inode->i_mapping);
> +
> + *pfn_ops = &shmem_pfn_ops;
> + return 0;
> +}
> +
> +void shmem_unregister_falloc_notifier(struct inode *inode)
> +{
> + struct shmem_inode_info *info = SHMEM_I(inode);
> +
> + spin_lock(&info->lock);
> + info->owner = NULL;
> + info->falloc_notifier = NULL;
> + spin_unlock(&info->lock);
> +}
> +#endif
> +
> static const struct file_operations shmem_file_operations = {
> .mmap = shmem_mmap,
> .get_unmapped_area = shmem_get_unmapped_area,

Next message: Chao Peng: "Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast memslot lookup by file offset"
Previous message: Kai-Heng Feng: "[PATCH] ALSA: hda/realtek: Use ALC285_FIXUP_HP_GPIO_LED on another HP laptop"
In reply to: Chao Peng: "[PATCH v3 kvm/queue 03/16] mm/memfd: Introduce MEMFD_OPS"
Next in thread: Chao Peng: "[PATCH v3 kvm/queue 04/16] KVM: Extend the memslot to support fd-based private memory"
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