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48 lines
2.3 KiB
Plaintext
48 lines
2.3 KiB
Plaintext
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# Copyright (C) 2010-2021 Junjiro R. Okajima
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#
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 2 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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Dynamically customizable FS operations
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----------------------------------------------------------------------
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Generally FS operations (struct inode_operations, struct
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address_space_operations, struct file_operations, etc.) are defined as
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"static const", but it never means that FS have only one set of
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operation. Some FS have multiple sets of them. For instance, ext2 has
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three sets, one for XIP, for NOBH, and for normal.
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Since aufs overrides and redirects these operations, sometimes aufs has
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to change its behaviour according to the branch FS type. More importantly
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VFS acts differently if a function (member in the struct) is set or
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not. It means aufs should have several sets of operations and select one
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among them according to the branch FS definition.
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In order to solve this problem and not to affect the behaviour of VFS,
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aufs defines these operations dynamically. For instance, aufs defines
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dummy direct_IO function for struct address_space_operations, but it may
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not be set to the address_space_operations actually. When the branch FS
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doesn't have it, aufs doesn't set it to its address_space_operations
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while the function definition itself is still alive. So the behaviour
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itself will not change, and it will return an error when direct_IO is
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not set.
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The lifetime of these dynamically generated operation object is
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maintained by aufs branch object. When the branch is removed from aufs,
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the reference counter of the object is decremented. When it reaches
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zero, the dynamically generated operation object will be freed.
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This approach is designed to support AIO (io_submit), Direct I/O and
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XIP (DAX) mainly.
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Currently this approach is applied to address_space_operations for
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regular files only.
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