diff mbox series

[v1,5/5] migration/ram: Optimize ram_write_tracking_start() for RamDiscardManager

Message ID 20230105124528.93813-6-david@redhat.com
State New
Headers show
Series migration/ram: background snapshot fixes and optimiations | expand

Commit Message

David Hildenbrand Jan. 5, 2023, 12:45 p.m. UTC
ram_block_populate_read() already optimizes for RamDiscardManager.
However, ram_write_tracking_start() will still try protecting discarded
memory ranges.

Let's optimize, because discarded ranges don't map any pages and

(1) For anonymous memory, trying to protect using uffd-wp without a mapped
    page is ignored by the kernel and consequently a NOP.

(2) For shared/file-backed memory, we will fill present page tables in the
    range with PTE markers. However, we will even allocate page tables
    just to fill them with unnecessary PTE markers and effectively
    waste memory.

So let's exclude these ranges, just like ram_block_populate_read()
already does.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 migration/ram.c | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

Comments

Juan Quintela Feb. 2, 2023, 11:20 a.m. UTC | #1
David Hildenbrand <david@redhat.com> wrote:
> ram_block_populate_read() already optimizes for RamDiscardManager.
> However, ram_write_tracking_start() will still try protecting discarded
> memory ranges.
>
> Let's optimize, because discarded ranges don't map any pages and
>
> (1) For anonymous memory, trying to protect using uffd-wp without a mapped
>     page is ignored by the kernel and consequently a NOP.
>
> (2) For shared/file-backed memory, we will fill present page tables in the
>     range with PTE markers. However, we will even allocate page tables
>     just to fill them with unnecessary PTE markers and effectively
>     waste memory.
>
> So let's exclude these ranges, just like ram_block_populate_read()
> already does.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>
diff mbox series

Patch

diff --git a/migration/ram.c b/migration/ram.c
index 73a443f683..50ee1fa147 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1865,6 +1865,39 @@  void ram_write_tracking_prepare(void)
     }
 }
 
+static inline int uffd_protect_section(MemoryRegionSection *section,
+                                       void *opaque)
+{
+    const hwaddr size = int128_get64(section->size);
+    const hwaddr offset = section->offset_within_region;
+    RAMBlock *rb = section->mr->ram_block;
+    int uffd_fd = (uintptr_t)opaque;
+
+    return uffd_change_protection(uffd_fd, rb->host + offset, size, true,
+                                  false);
+}
+
+static int ram_block_uffd_protect(RAMBlock *rb, int uffd_fd)
+{
+    assert(rb->flags & RAM_UF_WRITEPROTECT);
+
+    /* See ram_block_populate_read() */
+    if (rb->mr && memory_region_has_ram_discard_manager(rb->mr)) {
+        RamDiscardManager *rdm = memory_region_get_ram_discard_manager(rb->mr);
+        MemoryRegionSection section = {
+            .mr = rb->mr,
+            .offset_within_region = 0,
+            .size = rb->mr->size,
+        };
+
+        return ram_discard_manager_replay_populated(rdm, &section,
+                                                    uffd_protect_section,
+                                                    (void *)(uintptr_t)uffd_fd);
+    }
+    return uffd_change_protection(uffd_fd, rb->host,
+                                  rb->used_length, true, false);
+}
+
 /*
  * ram_write_tracking_start: start UFFD-WP memory tracking
  *
@@ -1900,8 +1933,7 @@  int ram_write_tracking_start(void)
         memory_region_ref(block->mr);
 
         /* Apply UFFD write protection to the block memory range */
-        if (uffd_change_protection(rs->uffdio_fd, block->host,
-                                   block->used_length, true, false)) {
+        if (ram_block_uffd_protect(block, uffd_fd)) {
             goto fail;
         }