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In this sync readahead function, since the index is 0, it will proceed to `initial_readahead` and initialize `ra_state`. It allocates a folio with an order of 2 and marks it as PG_readahead. Next, because` (folio_test_readahead(folio))` is true, the page_cache_async_ra function is called, which causes the `ra_state` to be initialized again.