system/physmem: take into account fd_offset for file fallocate

Punching a hole in a file with fallocate needs to take into account the
fd_offset value for a correct file location.
But guest_memfd internal use doesn't currently consider fd_offset.

Fixes: 4b870dc ("hostmem-file: add offset option")

Signed-off-by: William Roche <william.roche@oracle.com>

system/physmem.c

@@ -3655,6 +3655,7 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
         need_madvise = (rb->page_size == qemu_real_host_page_size());
         need_fallocate = rb->fd != -1;
         if (need_fallocate) {
+            uint64_t file_offset = start + rb->fd_offset;
             /* 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.
@@ -3689,18 +3690,18 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
             }
 
             ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-                            start, length);
+                            file_offset, length);
             if (ret) {
                 ret = -errno;
                 error_report("%s: Failed to fallocate %s:%" PRIx64 " +%zx (%d)",
-                             __func__, rb->idstr, start, length, ret);
+                             __func__, rb->idstr, file_offset, length, ret);
                 goto err;
             }
 #else
             ret = -ENOSYS;
             error_report("%s: fallocate not available/file"
                          "%s:%" PRIx64 " +%zx (%d)",
-                         __func__, rb->idstr, start, length, ret);
+                         __func__, rb->idstr, file_offset, length, ret);
             goto err;
 #endif
         }
@@ -3747,6 +3748,7 @@ int ram_block_discard_guest_memfd_range(RAMBlock *rb, uint64_t start,
     int ret = -1;
 
 #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
+    /* ignore fd_offset with guest_memfd */
     ret = fallocate(rb->guest_memfd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
                     start, length);