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Kernel: Introduce support for using FileSystem object in multiple mounts

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The idea is to enable mounting FileSystem objects across multiple mounts
in contrast to what happened until now - each mount has its own unique
FileSystem object being attached to it.

Considering a situation of mounting a block device at 2 different mount
points at in system, there were a couple of critical flaws due to how
the previous "design" worked:
1. BlockBasedFileSystem(s) that pointed to the same actual device had a
separate DiskCache object being attached to them. Because both instances
were not synchronized by any means, corruption of the filesystem is most
likely achieveable by a simple cache flush of either of the instances.
2. For superblock-oriented filesystems (such as the ext2 filesystem),
lack of synchronization between both instances can lead to severe
corruption in the superblock, which could render the entire filesystem
unusable.
3. Flags of a specific filesystem implementation (for example, with xfs
on Linux, one can instruct to mount it with the discard option) must be
honored across multiple mounts, to ensure expected behavior against a
particular filesystem.

This patch put the foundations to start fix the issues mentioned above.
However, there are still major issues to solve, so this is only a start.
This commit is contained in:
Liav A 2022-08-20 00:03:24 +03:00 • committed by Andrew Kaster
parent 965afba320
commit 0fd7b688af
17 changed files with 168 additions and 33 deletions
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8
Kernel/FileSystem/FileSystem.h
View file

@ -41,7 +41,7 @@ public:
virtual unsigned total_inode_count() const { return 0; }
virtual unsigned free_inode_count() const { return 0; }
virtual ErrorOr<void> prepare_to_unmount() { return {}; }
ErrorOr<void> prepare_to_unmount();
struct DirectoryEntryView {
DirectoryEntryView(StringView name, InodeIdentifier, u8 file_type);
@ -61,12 +61,16 @@ public:
// Converts file types that are used internally by the filesystem to DT_* types
virtual u8 internal_file_type_to_directory_entry_type(DirectoryEntryView const& entry) const { return entry.file_type; }
SpinlockProtected<size_t>& mounted_count(Badge<VirtualFileSystem>) { return m_attach_count; }
protected:
FileSystem();
void set_block_size(u64 size) { m_block_size = size; }
void set_fragment_size(size_t size) { m_fragment_size = size; }
virtual ErrorOr<void> prepare_to_clear_last_mount() { return {}; }
mutable Mutex m_lock { "FS"sv };
private:
@ -74,6 +78,8 @@ private:
u64 m_block_size { 0 };
size_t m_fragment_size { 0 };
bool m_readonly { false };
SpinlockProtected<size_t> m_attach_count { LockRank::FileSystem, 0 };
};
inline FileSystem* InodeIdentifier::fs() // NOLINT(readability-make-member-function-const) const InodeIdentifiers should not be able to modify the FileSystem