[Qemu-devel] [RFC v2 03/32] migrate: Update ram_block_discard_range for shared

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

The choice of call to discard a block is getting more complicated
for other cases.   We use fallocate PUNCH_HOLE in any file cases;
it works for both hugepage and for tmpfs.
We use the DONTNEED for non-hugepage cases either where they're
anonymous or where they're private.

Care should be taken when trying other backing files.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 exec.c       | 35 ++++++++++++++++++++++++-----------
 trace-events |  3 +++
 2 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/exec.c b/exec.c
index d20c34ca83..67df2909ce 100644
--- a/exec.c
+++ b/exec.c
@@ -3573,6 +3573,7 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
     }
 
     if ((start + length) <= rb->used_length) {
+        bool need_madvise, need_fallocate;
         uint8_t *host_endaddr = host_startaddr + length;
         if ((uintptr_t)host_endaddr & (rb->page_size - 1)) {
             error_report("ram_block_discard_range: Unaligned end address: %p",
@@ -3582,23 +3583,35 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
 
         errno = ENOTSUP; /* If we are missing MADVISE etc */
 
-        if (rb->page_size == qemu_host_page_size) {
-#if defined(CONFIG_MADVISE)
-            /* Note: We need the madvise MADV_DONTNEED behaviour of definitely
-             * freeing the page.
-             */
-            ret = madvise(host_startaddr, length, MADV_DONTNEED);
-#endif
-        } else {
-            /* Huge page case  - unfortunately it can't do DONTNEED, but
-             * it can do the equivalent by FALLOC_FL_PUNCH_HOLE in the
-             * huge page file.
+        /* The logic here is messy;
+         *    madvise DONTNEED fails for hugepages
+         *    fallocate works on hugepages and shmem
+         */
+        need_madvise = (rb->page_size == qemu_host_page_size);
+        need_fallocate = rb->fd != -1;
+        if (need_fallocate) {
+            /* For a file, this causes the area of the file to be zero'd
+             * if read, and for hugetlbfs also causes it to be unmapped
+             * so a userfault will trigger.
              */
 #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
             ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
                             start, length);
 #endif
         }
+        /* i.e. need madvise but skip it if the fallocate failed */
+        if (need_madvise && (!need_fallocate || (ret == 0))) {
+            /* For normal RAM this causes it to be unmapped,
+             * for shared memory it causes the local mapping to disappear
+             * and to fall back on the file contents (which we just
+             * fallocate'd away).
+             */
+#if defined(CONFIG_MADVISE)
+            ret =  madvise(host_startaddr, length, MADV_DONTNEED);
+#endif
+        }
+        trace_ram_block_discard_range(rb->idstr, host_startaddr,
+                                      need_madvise, need_fallocate, ret);
         if (ret) {
             ret = -errno;
             error_report("ram_block_discard_range: Failed to discard range "
diff --git a/trace-events b/trace-events
index 1f50f56d9d..213ee34f89 100644
--- a/trace-events
+++ b/trace-events
@@ -55,6 +55,9 @@ dma_complete(void *dbs, int ret, void *cb) "dbs=%p ret=%d cb=%p"
 dma_blk_cb(void *dbs, int ret) "dbs=%p ret=%d"
 dma_map_wait(void *dbs) "dbs=%p"
 
+# exec.c
+ram_block_discard_range(const char *rbname, void *hva, bool need_madvise, bool need_fallocate, int ret) "%s@%p: madvise: %d fallocate: %d ret: %d"
+
 # memory.c
 memory_region_ops_read(int cpu_index, void *mr, uint64_t addr, uint64_t value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" size %u"
 memory_region_ops_write(int cpu_index, void *mr, uint64_t addr, uint64_t value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" size %u"
-- 
2.13.5