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Vladimir Davydov authored
The limit equals 32 and is imposed by the number of entries in the fs_poolid_map and shared_fs_poolid_map. Nowadays it is insufficient, because with containers on board a Linux host can have hundreds of active fs mounts. These maps were introduced by commit 49a9ab81 ("mm: cleancache: lazy initialization to allow tmem backends to build/run as modules") in order to allow compiling cleancache drivers as modules. Real pool ids are stored in these maps while super_block->cleancache_poolid points to an entry in the map, so that on cleancache registration we can walk over all (if there are <= 32 of them, of course) cleancache-enabled super blocks and assign real pool ids. Actually, there is absolutely no need in these maps, because we can iterate over all super blocks immediately using iterate_supers. This is not racy, because cleancache_init_ops is called from mount_fs with super_block->s_umount held for writing, while iterate_su...
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