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