From 2bad466cc9d9b4c3b4b16eb9c03c919b59561316 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Thu, 9 Mar 2023 17:37:10 -0500 Subject: mm/uffd: UFFD_FEATURE_WP_UNPOPULATED Patch series "mm/uffd: Add feature bit UFFD_FEATURE_WP_UNPOPULATED", v4. The new feature bit makes anonymous memory acts the same as file memory on userfaultfd-wp in that it'll also wr-protect none ptes. It can be useful in two cases: (1) Uffd-wp app that needs to wr-protect none ptes like QEMU snapshot, so pre-fault can be replaced by enabling this flag and speed up protections (2) It helps to implement async uffd-wp mode that Muhammad is working on [1] It's debatable whether this is the most ideal solution because with the new feature bit set, wr-protect none pte needs to pre-populate the pgtables to the last level (PAGE_SIZE). But it seems fine so far to service either purpose above, so we can leave optimizations for later. The series brings pte markers to anonymous memory too. There's some change in the common mm code path in the 1st patch, great to have some eye looking at it, but hopefully they're still relatively straightforward. This patch (of 2): This is a new feature that controls how uffd-wp handles none ptes. When it's set, the kernel will handle anonymous memory the same way as file memory, by allowing the user to wr-protect unpopulated ptes. File memories handles none ptes consistently by allowing wr-protecting of none ptes because of the unawareness of page cache being exist or not. For anonymous it was not as persistent because we used to assume that we don't need protections on none ptes or known zero pages. One use case of such a feature bit was VM live snapshot, where if without wr-protecting empty ptes the snapshot can contain random rubbish in the holes of the anonymous memory, which can cause misbehave of the guest when the guest OS assumes the pages should be all zeros. QEMU worked it around by pre-populate the section with reads to fill in zero page entries before starting the whole snapshot process [1]. Recently there's another need raised on using userfaultfd wr-protect for detecting dirty pages (to replace soft-dirty in some cases) [2]. In that case if without being able to wr-protect none ptes by default, the dirty info can get lost, since we cannot treat every none pte to be dirty (the current design is identify a page dirty based on uffd-wp bit being cleared). In general, we want to be able to wr-protect empty ptes too even for anonymous. This patch implements UFFD_FEATURE_WP_UNPOPULATED so that it'll make uffd-wp handling on none ptes being consistent no matter what the memory type is underneath. It doesn't have any impact on file memories so far because we already have pte markers taking care of that. So it only affects anonymous. The feature bit is by default off, so the old behavior will be maintained. Sometimes it may be wanted because the wr-protect of none ptes will contain overheads not only during UFFDIO_WRITEPROTECT (by applying pte markers to anonymous), but also on creating the pgtables to store the pte markers. So there's potentially less chance of using thp on the first fault for a none pmd or larger than a pmd. The major implementation part is teaching the whole kernel to understand pte markers even for anonymously mapped ranges, meanwhile allowing the UFFDIO_WRITEPROTECT ioctl to apply pte markers for anonymous too when the new feature bit is set. Note that even if the patch subject starts with mm/uffd, there're a few small refactors to major mm path of handling anonymous page faults. But they should be straightforward. With WP_UNPOPUATED, application like QEMU can avoid pre-read faults all the memory before wr-protect during taking a live snapshot. Quotting from Muhammad's test result here [3] based on a simple program [4]: (1) With huge page disabled echo madvise > /sys/kernel/mm/transparent_hugepage/enabled ./uffd_wp_perf Test DEFAULT: 4 Test PRE-READ: 1111453 (pre-fault 1101011) Test MADVISE: 278276 (pre-fault 266378) Test WP-UNPOPULATE: 11712 (2) With Huge page enabled echo always > /sys/kernel/mm/transparent_hugepage/enabled ./uffd_wp_perf Test DEFAULT: 4 Test PRE-READ: 22521 (pre-fault 22348) Test MADVISE: 4909 (pre-fault 4743) Test WP-UNPOPULATE: 14448 There'll be a great perf boost for no-thp case, while for thp enabled with extreme case of all-thp-zero WP_UNPOPULATED can be slower than MADVISE, but that's low possibility in reality, also the overhead was not reduced but postponed until a follow up write on any huge zero thp, so potentially it is faster by making the follow up writes slower. [1] https://lore.kernel.org/all/20210401092226.102804-4-andrey.gruzdev@virtuozzo.com/ [2] https://lore.kernel.org/all/Y+v2HJ8+3i%2FKzDBu@x1n/ [3] https://lore.kernel.org/all/d0eb0a13-16dc-1ac1-653a-78b7273781e3@collabora.com/ [4] https://github.com/xzpeter/clibs/blob/master/uffd-test/uffd-wp-perf.c [peterx@redhat.com: comment changes, oneliner fix to khugepaged] Link: https://lkml.kernel.org/r/ZB2/8jPhD3fpx5U8@x1n Link: https://lkml.kernel.org/r/20230309223711.823547-1-peterx@redhat.com Link: https://lkml.kernel.org/r/20230309223711.823547-2-peterx@redhat.com Signed-off-by: Peter Xu Acked-by: David Hildenbrand Cc: Andrea Arcangeli Cc: Axel Rasmussen Cc: Mike Rapoport Cc: Muhammad Usama Anjum Cc: Nadav Amit Cc: Paul Gofman Signed-off-by: Andrew Morton --- include/linux/mm_inline.h | 6 ++++++ include/linux/userfaultfd_k.h | 23 +++++++++++++++++++++++ include/uapi/linux/userfaultfd.h | 10 +++++++++- 3 files changed, 38 insertions(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h index de1e622dd366..0e1d239a882c 100644 --- a/include/linux/mm_inline.h +++ b/include/linux/mm_inline.h @@ -557,6 +557,12 @@ pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr, /* The current status of the pte should be "cleared" before calling */ WARN_ON_ONCE(!pte_none(*pte)); + /* + * NOTE: userfaultfd_wp_unpopulated() doesn't need this whole + * thing, because when zapping either it means it's dropping the + * page, or in TTU where the present pte will be quickly replaced + * with a swap pte. There's no way of leaking the bit. + */ if (vma_is_anonymous(vma) || !userfaultfd_wp(vma)) return; diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h index 3767f18114ef..0cf8880219da 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -179,6 +179,7 @@ extern int userfaultfd_unmap_prep(struct mm_struct *mm, unsigned long start, unsigned long end, struct list_head *uf); extern void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf); +extern bool userfaultfd_wp_unpopulated(struct vm_area_struct *vma); #else /* CONFIG_USERFAULTFD */ @@ -274,8 +275,30 @@ static inline bool uffd_disable_fault_around(struct vm_area_struct *vma) return false; } +static inline bool userfaultfd_wp_unpopulated(struct vm_area_struct *vma) +{ + return false; +} + #endif /* CONFIG_USERFAULTFD */ +static inline bool userfaultfd_wp_use_markers(struct vm_area_struct *vma) +{ + /* Only wr-protect mode uses pte markers */ + if (!userfaultfd_wp(vma)) + return false; + + /* File-based uffd-wp always need markers */ + if (!vma_is_anonymous(vma)) + return true; + + /* + * Anonymous uffd-wp only needs the markers if WP_UNPOPULATED + * enabled (to apply markers on zero pages). + */ + return userfaultfd_wp_unpopulated(vma); +} + static inline bool pte_marker_entry_uffd_wp(swp_entry_t entry) { #ifdef CONFIG_PTE_MARKER_UFFD_WP diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h index 005e5e306266..90c958952bfc 100644 --- a/include/uapi/linux/userfaultfd.h +++ b/include/uapi/linux/userfaultfd.h @@ -38,7 +38,8 @@ UFFD_FEATURE_MINOR_HUGETLBFS | \ UFFD_FEATURE_MINOR_SHMEM | \ UFFD_FEATURE_EXACT_ADDRESS | \ - UFFD_FEATURE_WP_HUGETLBFS_SHMEM) + UFFD_FEATURE_WP_HUGETLBFS_SHMEM | \ + UFFD_FEATURE_WP_UNPOPULATED) #define UFFD_API_IOCTLS \ ((__u64)1 << _UFFDIO_REGISTER | \ (__u64)1 << _UFFDIO_UNREGISTER | \ @@ -203,6 +204,12 @@ struct uffdio_api { * * UFFD_FEATURE_WP_HUGETLBFS_SHMEM indicates that userfaultfd * write-protection mode is supported on both shmem and hugetlbfs. + * + * UFFD_FEATURE_WP_UNPOPULATED indicates that userfaultfd + * write-protection mode will always apply to unpopulated pages + * (i.e. empty ptes). This will be the default behavior for shmem + * & hugetlbfs, so this flag only affects anonymous memory behavior + * when userfault write-protection mode is registered. */ #define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0) #define UFFD_FEATURE_EVENT_FORK (1<<1) @@ -217,6 +224,7 @@ struct uffdio_api { #define UFFD_FEATURE_MINOR_SHMEM (1<<10) #define UFFD_FEATURE_EXACT_ADDRESS (1<<11) #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM (1<<12) +#define UFFD_FEATURE_WP_UNPOPULATED (1<<13) __u64 features; __u64 ioctls; -- cgit v1.2.3