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In a transaction, allocator might need to allocate space from an empty non-evict-able page which isn’t in cache yet, then it could call the umem_cache_map() to map the empty MD page to a free memory page, to avoid NVMe I/O (CPU yield) in umem_cache_map() , we must ensure that there are is at least one free in-memory pages page (not in-use, not dirty) available for mapping, this can be achieved by calling umem_cache_reserve() before starting the local transaction.
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/**
* Reserve few free memory pages for the potential non-evictable page growing. It's
* usually called before starting a transaction, and the transaction might need to
* alloc space from empty non-evictable pages which are not mapped yet.
*
* \param[in] store UMEM store
*
* \return Zero on success, negative value on error.
*/
int umem_cache_reserve(struct umem_store *store); |
To handle the potential umem_cache_map() call within a transaction, umem_cache_reserve() will be called before starting a non-nested transaction (in vos_wal_reserve()), it checks the number of non-evict-able pages in cache, if it haven't reached the maximum value (see umem_cache_alloc()), few free in-memory pages (not in-use, not dirty) will be reserved.
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/** * Load & Map MD pages in specified range to memory pages, then take a * reference on the mapped memory pages, so that the pages won't be evicted * until unpin is called. It's usually for the cases where we need the pages * stay loaded across a yield. * * \param[in] store UMEM store * \param[in] ranges Ranges to be pinned * \param[in] range_nr Number of ranges * \param[in] is_sys Internal access from system ULTs (aggregation etc.) * * \return Zero on success, negative value on error */ int umem_cache_pin(struct umem_store *store, umem_cache_pin_range *ranges, int range_nr, bool is_sys); /** * Unpin the pages pinned by prior umem_cache_pin(). * * \param[in] store UMEM store * \param[in] ranges Ranges to be pinned * \param[in] range_nr Number of ranges * * \return N/A */ void umem_cache_unpin(struct umem_store *store, umem_cache_pin_range *ranges, int range_nr); |
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