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SubjectRe: [PATCH v20 2/5] ring-buffer: Introducing ring-buffer mapping functions
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On 19.04.24 20:25, David Hildenbrand wrote:
> On 06.04.24 19:36, Vincent Donnefort wrote:
>> In preparation for allowing the user-space to map a ring-buffer, add
>> a set of mapping functions:
>>
>> ring_buffer_{map,unmap}()
>>
>> And controls on the ring-buffer:
>>
>> ring_buffer_map_get_reader() /* swap reader and head */
>>
>> Mapping the ring-buffer also involves:
>>
>> A unique ID for each subbuf of the ring-buffer, currently they are
>> only identified through their in-kernel VA.
>>
>> A meta-page, where are stored ring-buffer statistics and a
>> description for the current reader
>>
>> The linear mapping exposes the meta-page, and each subbuf of the
>> ring-buffer, ordered following their unique ID, assigned during the
>> first mapping.
>>
>> Once mapped, no subbuf can get in or out of the ring-buffer: the buffer
>> size will remain unmodified and the splice enabling functions will in
>> reality simply memcpy the data instead of swapping subbufs.
>>
>> CC: <linux-mm@kvack.org>
>> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
>>
>> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
>> index dc5ae4e96aee..96d2140b471e 100644
>> --- a/include/linux/ring_buffer.h
>> +++ b/include/linux/ring_buffer.h
>> @@ -6,6 +6,8 @@
>> #include <linux/seq_file.h>
>> #include <linux/poll.h>
>>
>> +#include <uapi/linux/trace_mmap.h>
>> +
>> struct trace_buffer;
>> struct ring_buffer_iter;
>>
>> @@ -223,4 +225,8 @@ int trace_rb_cpu_prepare(unsigned int cpu, struct hlist_node *node);
>> #define trace_rb_cpu_prepare NULL
>> #endif
>>
>> +int ring_buffer_map(struct trace_buffer *buffer, int cpu,
>> + struct vm_area_struct *vma);
>> +int ring_buffer_unmap(struct trace_buffer *buffer, int cpu);
>> +int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu);
>> #endif /* _LINUX_RING_BUFFER_H */
>> diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
>> new file mode 100644
>> index 000000000000..ffcd8dfcaa4f
>> --- /dev/null
>> +++ b/include/uapi/linux/trace_mmap.h
>> @@ -0,0 +1,46 @@
>> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
>> +#ifndef _TRACE_MMAP_H_
>> +#define _TRACE_MMAP_H_
>> +
>> +#include <linux/types.h>
>> +
>> +/**
>> + * struct trace_buffer_meta - Ring-buffer Meta-page description
>> + * @meta_page_size: Size of this meta-page.
>> + * @meta_struct_len: Size of this structure.
>> + * @subbuf_size: Size of each sub-buffer.
>> + * @nr_subbufs: Number of subbfs in the ring-buffer, including the reader.
>> + * @reader.lost_events: Number of events lost at the time of the reader swap.
>> + * @reader.id: subbuf ID of the current reader. ID range [0 : @nr_subbufs - 1]
>> + * @reader.read: Number of bytes read on the reader subbuf.
>> + * @flags: Placeholder for now, 0 until new features are supported.
>> + * @entries: Number of entries in the ring-buffer.
>> + * @overrun: Number of entries lost in the ring-buffer.
>> + * @read: Number of entries that have been read.
>> + * @Reserved1: Reserved for future use.
>> + * @Reserved2: Reserved for future use.
>> + */
>> +struct trace_buffer_meta {
>> + __u32 meta_page_size;
>> + __u32 meta_struct_len;
>> +
>> + __u32 subbuf_size;
>> + __u32 nr_subbufs;
>> +
>> + struct {
>> + __u64 lost_events;
>> + __u32 id;
>> + __u32 read;
>> + } reader;
>> +
>> + __u64 flags;
>> +
>> + __u64 entries;
>> + __u64 overrun;
>> + __u64 read;
>> +
>> + __u64 Reserved1;
>> + __u64 Reserved2;
>> +};
>> +
>> +#endif /* _TRACE_MMAP_H_ */
>> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
>> index cc9ebe593571..793ecc454039 100644
>> --- a/kernel/trace/ring_buffer.c
>> +++ b/kernel/trace/ring_buffer.c
>> @@ -9,6 +9,7 @@
>> #include <linux/ring_buffer.h>
>> #include <linux/trace_clock.h>
>> #include <linux/sched/clock.h>
>> +#include <linux/cacheflush.h>
>> #include <linux/trace_seq.h>
>> #include <linux/spinlock.h>
>> #include <linux/irq_work.h>
>> @@ -26,6 +27,7 @@
>> #include <linux/list.h>
>> #include <linux/cpu.h>
>> #include <linux/oom.h>
>> +#include <linux/mm.h>
>>
>> #include <asm/local64.h>
>> #include <asm/local.h>
>> @@ -338,6 +340,7 @@ struct buffer_page {
>> local_t entries; /* entries on this page */
>> unsigned long real_end; /* real end of data */
>> unsigned order; /* order of the page */
>> + u32 id; /* ID for external mapping */
>> struct buffer_data_page *page; /* Actual data page */
>> };
>>
>> @@ -484,6 +487,12 @@ struct ring_buffer_per_cpu {
>> u64 read_stamp;
>> /* pages removed since last reset */
>> unsigned long pages_removed;
>> +
>> + unsigned int mapped;
>> + struct mutex mapping_lock;
>> + unsigned long *subbuf_ids; /* ID to subbuf VA */
>> + struct trace_buffer_meta *meta_page;
>> +
>> /* ring buffer pages to update, > 0 to add, < 0 to remove */
>> long nr_pages_to_update;
>> struct list_head new_pages; /* new pages to add */
>> @@ -1599,6 +1608,7 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
>> init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
>> init_waitqueue_head(&cpu_buffer->irq_work.waiters);
>> init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
>> + mutex_init(&cpu_buffer->mapping_lock);
>>
>> bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
>> GFP_KERNEL, cpu_to_node(cpu));
>> @@ -1789,8 +1799,6 @@ bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer)
>> return buffer->time_stamp_abs;
>> }
>>
>> -static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);
>> -
>> static inline unsigned long rb_page_entries(struct buffer_page *bpage)
>> {
>> return local_read(&bpage->entries) & RB_WRITE_MASK;
>> @@ -5211,6 +5219,22 @@ static void rb_clear_buffer_page(struct buffer_page *page)
>> page->read = 0;
>> }
>>
>> +static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
>> +{
>> + struct trace_buffer_meta *meta = cpu_buffer->meta_page;
>> +
>> + meta->reader.read = cpu_buffer->reader_page->read;
>> + meta->reader.id = cpu_buffer->reader_page->id;
>> + meta->reader.lost_events = cpu_buffer->lost_events;
>> +
>> + meta->entries = local_read(&cpu_buffer->entries);
>> + meta->overrun = local_read(&cpu_buffer->overrun);
>> + meta->read = cpu_buffer->read;
>> +
>> + /* Some archs do not have data cache coherency between kernel and user-space */
>> + flush_dcache_folio(virt_to_folio(cpu_buffer->meta_page));
>> +}
>> +
>> static void
>> rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
>> {
>> @@ -5255,6 +5279,9 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
>> cpu_buffer->lost_events = 0;
>> cpu_buffer->last_overrun = 0;
>>
>> + if (cpu_buffer->mapped)
>> + rb_update_meta_page(cpu_buffer);
>> +
>> rb_head_page_activate(cpu_buffer);
>> cpu_buffer->pages_removed = 0;
>> }
>> @@ -5469,6 +5496,12 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
>> cpu_buffer_a = buffer_a->buffers[cpu];
>> cpu_buffer_b = buffer_b->buffers[cpu];
>>
>> + /* It's up to the callers to not try to swap mapped buffers */
>> + if (WARN_ON_ONCE(cpu_buffer_a->mapped || cpu_buffer_b->mapped)) {
>> + ret = -EBUSY;
>> + goto out;
>> + }
>> +
>> /* At least make sure the two buffers are somewhat the same */
>> if (cpu_buffer_a->nr_pages != cpu_buffer_b->nr_pages)
>> goto out;
>> @@ -5733,7 +5766,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>> * Otherwise, we can simply swap the page with the one passed in.
>> */
>> if (read || (len < (commit - read)) ||
>> - cpu_buffer->reader_page == cpu_buffer->commit_page) {
>> + cpu_buffer->reader_page == cpu_buffer->commit_page ||
>> + cpu_buffer->mapped) {
>> struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
>> unsigned int rpos = read;
>> unsigned int pos = 0;
>> @@ -5956,6 +5990,11 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>>
>> cpu_buffer = buffer->buffers[cpu];
>>
>> + if (cpu_buffer->mapped) {
>> + err = -EBUSY;
>> + goto error;
>> + }
>> +
>> /* Update the number of pages to match the new size */
>> nr_pages = old_size * buffer->buffers[cpu]->nr_pages;
>> nr_pages = DIV_ROUND_UP(nr_pages, buffer->subbuf_size);
>> @@ -6057,6 +6096,358 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>> }
>> EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_set);
>>
>> +static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
>> +{
>> + struct page *page;
>> +
>> + if (cpu_buffer->meta_page)
>> + return 0;
>> +
>> + page = alloc_page(GFP_USER | __GFP_ZERO);
>> + if (!page)
>> + return -ENOMEM;
>> +
>> + cpu_buffer->meta_page = page_to_virt(page);
>> +
>> + return 0;
>> +}
>> +
>> +static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
>> +{
>> + unsigned long addr = (unsigned long)cpu_buffer->meta_page;
>> +
>> + free_page(addr);
>> + cpu_buffer->meta_page = NULL;
>> +}
>> +
>> +static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
>> + unsigned long *subbuf_ids)
>> +{
>> + struct trace_buffer_meta *meta = cpu_buffer->meta_page;
>> + unsigned int nr_subbufs = cpu_buffer->nr_pages + 1;
>> + struct buffer_page *first_subbuf, *subbuf;
>> + int id = 0;
>> +
>> + subbuf_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
>> + cpu_buffer->reader_page->id = id++;
>> +
>> + first_subbuf = subbuf = rb_set_head_page(cpu_buffer);
>> + do {
>> + if (WARN_ON(id >= nr_subbufs))
>> + break;
>> +
>> + subbuf_ids[id] = (unsigned long)subbuf->page;
>> + subbuf->id = id;
>> +
>> + rb_inc_page(&subbuf);
>> + id++;
>> + } while (subbuf != first_subbuf);
>> +
>> + /* install subbuf ID to kern VA translation */
>> + cpu_buffer->subbuf_ids = subbuf_ids;
>> +
>> + /* __rb_map_vma() pads the meta-page to align it with the sub-buffers */
>> + meta->meta_page_size = PAGE_SIZE << cpu_buffer->buffer->subbuf_order;
>> + meta->meta_struct_len = sizeof(*meta);
>> + meta->nr_subbufs = nr_subbufs;
>> + meta->subbuf_size = cpu_buffer->buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
>> +
>> + rb_update_meta_page(cpu_buffer);
>> +}
>> +
>> +static struct ring_buffer_per_cpu *
>> +rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
>> +{
>> + struct ring_buffer_per_cpu *cpu_buffer;
>> +
>> + if (!cpumask_test_cpu(cpu, buffer->cpumask))
>> + return ERR_PTR(-EINVAL);
>> +
>> + cpu_buffer = buffer->buffers[cpu];
>> +
>> + mutex_lock(&cpu_buffer->mapping_lock);
>> +
>> + if (!cpu_buffer->mapped) {
>> + mutex_unlock(&cpu_buffer->mapping_lock);
>> + return ERR_PTR(-ENODEV);
>> + }
>> +
>> + return cpu_buffer;
>> +}
>> +
>> +static void rb_put_mapped_buffer(struct ring_buffer_per_cpu *cpu_buffer)
>> +{
>> + mutex_unlock(&cpu_buffer->mapping_lock);
>> +}
>> +
>> +/*
>> + * Fast-path for rb_buffer_(un)map(). Called whenever the meta-page doesn't need
>> + * to be set-up or torn-down.
>> + */
>> +static int __rb_inc_dec_mapped(struct ring_buffer_per_cpu *cpu_buffer,
>> + bool inc)
>> +{
>> + unsigned long flags;
>> +
>> + lockdep_assert_held(&cpu_buffer->mapping_lock);
>> +
>> + if (inc && cpu_buffer->mapped == UINT_MAX)
>> + return -EBUSY;
>> +
>> + if (WARN_ON(!inc && cpu_buffer->mapped == 0))
>> + return -EINVAL;
>> +
>> + mutex_lock(&cpu_buffer->buffer->mutex);
>> + raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>> +
>> + if (inc)
>> + cpu_buffer->mapped++;
>> + else
>> + cpu_buffer->mapped--;
>> +
>> + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>> + mutex_unlock(&cpu_buffer->buffer->mutex);
>> +
>> + return 0;
>> +}
>> +
>> +#define subbuf_page(off, start) \
>> + virt_to_page((void *)((start) + ((off) << PAGE_SHIFT)))
>> +
>> +#define foreach_subbuf_page(sub_order, start, page) \
>> + page = subbuf_page(0, (start)); \
>> + for (int __off = 0; __off < (1 << (sub_order)); \
>> + __off++, page = subbuf_page(__off, (start)))
>> +
>> +/*
>> + * +--------------+ pgoff == 0
>> + * | meta page |
>> + * +--------------+ pgoff == 1
>> + * | 000000000 |
>> + * +--------------+ pgoff == (1 << subbuf_order)
>> + * | subbuffer 0 |
>> + * | |
>> + * +--------------+ pgoff == (2 * (1 << subbuf_order))
>> + * | subbuffer 1 |
>> + * | |
>> + * ...
>> + */
>> +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
>> + struct vm_area_struct *vma)
>> +{
>> + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
>> + unsigned int subbuf_pages, subbuf_order;
>> + struct page **pages;
>> + int p = 0, s = 0;
>> + int err;
>> +
>> + lockdep_assert_held(&cpu_buffer->mapping_lock);
>> +
>> + subbuf_order = cpu_buffer->buffer->subbuf_order;
>> + subbuf_pages = 1 << subbuf_order;
>> +
>> + if (subbuf_order && pgoff % subbuf_pages)
>> + return -EINVAL;
>> +
>> + nr_subbufs = cpu_buffer->nr_pages + 1;
>> + nr_pages = ((nr_subbufs + 1) << subbuf_order) - pgoff;
>> +
>> + vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
>> + if (!vma_pages || vma_pages > nr_pages)
>> + return -EINVAL;
>> +
>> + nr_pages = vma_pages;
>> +
>> + pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);
>> + if (!pages)
>> + return -ENOMEM;
>> +
>> + if (!pgoff) {
>> + unsigned long meta_page_padding;
>> +
>> + pages[p++] = virt_to_page(cpu_buffer->meta_page);
>> +
>> + /*
>> + * Pad with the zero-page to align the meta-page with the
>> + * sub-buffers.
>> + */
>> + meta_page_padding = subbuf_pages - 1;
>> + while (meta_page_padding-- && p < nr_pages)
>> + pages[p++] = ZERO_PAGE(0);
>
> Using the shared zeropage in a MAP_SHARED mapping that is neither VM_IO
> nor VM_PFNMAP can be problematic. So we really need patch #3 logic to
> use VM_PFNMAP.
>
> It would be cleaner/more obvious if these VMA flag setting would reside
> here, if possible.
>
>> + } else {
>> + /* Skip the meta-page */
>> + pgoff -= subbuf_pages;
>> +
>> + s += pgoff / subbuf_pages;
>> + }
>> +
>> + while (s < nr_subbufs && p < nr_pages) {
>> + struct page *page;
>> +
>> + foreach_subbuf_page(subbuf_order, cpu_buffer->subbuf_ids[s], page) {
>> + if (p >= nr_pages)
>> + break;
>> +
>> + pages[p++] = page;
>> + }
>> + s++;
>> + }
>> +
>> + err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
>
> I think Linus suggested it ("avoid all the sub-page ref-counts entirely
> by using VM_PFNMAP, and use vm_insert_pages()"), but ...
> vm_insert_pages() will:
> * Mess with mapcounts
> * Mess with refcounts
>
> See
> insert_pages()->insert_page_in_batch_locked()->insert_page_into_pte_locked().
>
> So we'll mess with the mapcount and refcount of the shared zeropage ...
> hmmmm
>
> If I am not wrong, vm_normal_page() would not return the shared zeropage
> in case we don't have CONFIG_ARCH_HAS_PTE_SPECIAL ... so
> unmap()->...->zap_present_ptes() would not decrement the refcount and we
> could overflow it. ... we also shouldn't ever mess with the mapcount of
> the shared zeropage in the first place.
>
> vm_insert_page() is clearer on that: "This allows drivers to insert
> individual pages they've allocated into a user vma". You didn't allocate
> the shared zeropage.
>
> I'm wondering if we even expect VM_MIXEDMAP and VM_PFNMAP to be set at
> the same time? vm_insert_pages() would BUG_ON in case VM_PFNMAP is
> already set and it would set VM_MIXEDMAP ... similarly
> vmf_insert_pfn_prot() would not be happy about that at all ...
>
> BUG_ON((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) ==
> (VM_PFNMAP|VM_MIXEDMAP));
>
> ... remap_pfn_range() is used by io_uring_mmap for a similar purpose.
> But it only supports a single PFN range at a time and requires the
> caller to handle refcounting of pages.
>
> It's getting late in Germany, so I might be missing something; but using
> the shared zeropage here might be a problem.
>

I was just about to write code to cleanly support vm_insert_pages() to
consume zeropages ... when I started to wonder about something else:


+ if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
+ !(vma->vm_flags & VM_MAYSHARE))
+ return -EPERM;
+

You disallow writable mappings. But what about using mprotect()
afterwards to allow for write permissions?

Likely you'd want to remove VM_MAYWRITE from the flags, otherwise
mprotect() might alter succeed.

--
Cheers,

David / dhildenb


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