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migration/ram: Optimize ram_write_tracking_start() for RamDiscardManager

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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.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
David Hildenbrand 2023-01-05 13:45:28 +01:00 committed by Juan Quintela
parent 59bcc049c1
commit e41c57702e
1 changed files with 34 additions and 2 deletions
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36
migration/ram.c
View file

@ -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;
}