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ring-buffer: Align meta-page to sub-buffers for improved TLB usage

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Previously, the mapped ring-buffer layout caused misalignment between
the meta-page and sub-buffers when the sub-buffer size was not a
multiple of PAGE_SIZE. This prevented hardware with larger TLB entries
from utilizing them effectively.

Add a padding with the zero-page between the meta-page and sub-buffers.
Also update the ring-buffer map_test to verify that padding.

Link: https://lore.kernel.org/20240628104611.1443542-1-vdonnefort@google.com
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Vincent Donnefort 2024-06-28 11:46:11 +01:00 committed by Steven Rostedt (Google)
parent d0f2d6e951
commit eb2dcde9f9
2 changed files with 34 additions and 13 deletions
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33
kernel/trace/ring_buffer.c
View File

@ -6852,10 +6852,10 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
/* install subbuf ID to kern VA translation */
cpu_buffer->subbuf_ids = subbuf_ids;
meta->meta_page_size = PAGE_SIZE;
meta->meta_struct_len = sizeof(*meta);
meta->nr_subbufs = nr_subbufs;
meta->subbuf_size = cpu_buffer->buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
meta->meta_page_size = meta->subbuf_size;
rb_update_meta_page(cpu_buffer);
}
@ -6949,6 +6949,12 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
!(vma->vm_flags & VM_MAYSHARE))
return -EPERM;
subbuf_order = cpu_buffer->buffer->subbuf_order;
subbuf_pages = 1 << subbuf_order;
if (subbuf_order && pgoff % subbuf_pages)
return -EINVAL;
/*
* Make sure the mapping cannot become writable later. Also tell the VM
* to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND).
@ -6958,11 +6964,8 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
lockdep_assert_held(&cpu_buffer->mapping_lock);
subbuf_order = cpu_buffer->buffer->subbuf_order;
subbuf_pages = 1 << subbuf_order;
nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
nr_pages = ((nr_subbufs + 1) << subbuf_order) - pgoff; /* + meta-page */
nr_vma_pages = vma_pages(vma);
if (!nr_vma_pages || nr_vma_pages > nr_pages)
@ -6975,20 +6978,24 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
return -ENOMEM;
if (!pgoff) {
unsigned long meta_page_padding;
pages[p++] = virt_to_page(cpu_buffer->meta_page);
/*
* TODO: Align sub-buffers on their size, once
* vm_insert_pages() supports the zero-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) {
unsigned long __maybe_unused zero_addr =
vma->vm_start + (PAGE_SIZE * p);
pages[p++] = ZERO_PAGE(zero_addr);
}
} else {
/* Skip the meta-page */
pgoff--;
if (pgoff % subbuf_pages) {
err = -EINVAL;
goto out;
}
pgoff -= subbuf_pages;
s += pgoff / subbuf_pages;
}

14
tools/testing/selftests/ring-buffer/map_test.c
View File

@ -228,6 +228,20 @@ TEST_F(map, data_mmap)
data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
desc->cpu_fd, meta_len);
ASSERT_EQ(data, MAP_FAILED);
/* Verify meta-page padding */
if (desc->meta->meta_page_size > getpagesize()) {
void *addr;
data_len = desc->meta->meta_page_size;
data = mmap(NULL, data_len,
PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
ASSERT_NE(data, MAP_FAILED);
addr = (void *)((unsigned long)data + getpagesize());
ASSERT_EQ(*((int *)addr), 0);
munmap(data, data_len);
}
}
FIXTURE(snapshot) {