| .. SPDX-License-Identifier: GPL-2.0 |
| |
| :Author: Chris Li <chrisl@kernel.org>, Kairui Song <kasong@tencent.com> |
| |
| ========== |
| Swap Table |
| ========== |
| |
| Swap table implements swap cache as a per-cluster swap cache value array. |
| |
| Swap Entry |
| ---------- |
| |
| A swap entry contains the information required to serve the anonymous page |
| fault. |
| |
| Swap entry is encoded as two parts: swap type and swap offset. |
| |
| The swap type indicates which swap device to use. |
| The swap offset is the offset of the swap file to read the page data from. |
| |
| Swap Cache |
| ---------- |
| |
| Swap cache is a map to look up folios using swap entry as the key. The result |
| value can have three possible types depending on which stage of this swap entry |
| was in. |
| |
| 1. NULL: This swap entry is not used. |
| |
| 2. folio: A folio has been allocated and bound to this swap entry. This is |
| the transient state of swap out or swap in. The folio data can be in |
| the folio or swap file, or both. |
| |
| 3. shadow: The shadow contains the working set information of the swapped |
| out folio. This is the normal state for a swapped out page. |
| |
| Swap Table Internals |
| -------------------- |
| |
| The previous swap cache is implemented by XArray. The XArray is a tree |
| structure. Each lookup will go through multiple nodes. Can we do better? |
| |
| Notice that most of the time when we look up the swap cache, we are either |
| in a swap in or swap out path. We should already have the swap cluster, |
| which contains the swap entry. |
| |
| If we have a per-cluster array to store swap cache value in the cluster. |
| Swap cache lookup within the cluster can be a very simple array lookup. |
| |
| We give such a per-cluster swap cache value array a name: the swap table. |
| |
| A swap table is an array of pointers. Each pointer is the same size as a |
| PTE. The size of a swap table for one swap cluster typically matches a PTE |
| page table, which is one page on modern 64-bit systems. |
| |
| With swap table, swap cache lookup can achieve great locality, simpler, |
| and faster. |
| |
| Locking |
| ------- |
| |
| Swap table modification requires taking the cluster lock. If a folio |
| is being added to or removed from the swap table, the folio must be |
| locked prior to the cluster lock. After adding or removing is done, the |
| folio shall be unlocked. |
| |
| Swap table lookup is protected by RCU and atomic read. If the lookup |
| returns a folio, the user must lock the folio before use. |
