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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
Notes: sideshowbarker 2024-07-17 08:43:11 +09:00
17 changed files with 168 additions and 33 deletions
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43
Kernel/FileSystem/VirtualFileSystem.cpp
View file

@ -65,7 +65,8 @@ ErrorOr<void> VirtualFileSystem::mount(FileSystem& fs, Custody& mount_point, int
auto new_mount = TRY(adopt_nonnull_own_or_enomem(new (nothrow) Mount(fs, &mount_point, flags)));
return m_mounts.with([&](auto& mounts) -> ErrorOr<void> {
auto& inode = mount_point.inode();
dbgln("VirtualFileSystem: Mounting {} at inode {} with flags {}",
dbgln("VirtualFileSystem: FileSystemID {}, Mounting {} at inode {} with flags {}",
fs.fsid(),
fs.class_name(),
inode.identifier(),
flags);
@ -73,6 +74,10 @@ ErrorOr<void> VirtualFileSystem::mount(FileSystem& fs, Custody& mount_point, int
dbgln("VirtualFileSystem: Mounting unsuccessful - inode {} is already a mount-point.", inode.identifier());
return EBUSY;
}
// Note: Actually add a mount for the filesystem and increment the filesystem mounted count
new_mount->guest_fs().mounted_count({}).with([&](auto& mounted_count) {
mounted_count++;
});
mounts.append(move(new_mount));
return {};
});
@ -106,16 +111,24 @@ ErrorOr<void> VirtualFileSystem::remount(Custody& mount_point, int new_flags)
return {};
}
ErrorOr<void> VirtualFileSystem::unmount(Inode& guest_inode)
ErrorOr<void> VirtualFileSystem::unmount(Custody& mountpoint_custody)
{
dbgln("VirtualFileSystem: unmount called with inode {}", guest_inode.identifier());
auto& guest_inode = mountpoint_custody.inode();
auto custody_path = TRY(mountpoint_custody.try_serialize_absolute_path());
dbgln("VirtualFileSystem: unmount called with inode {} on mountpoint {}", guest_inode.identifier(), custody_path->view());
return m_mounts.with([&](auto& mounts) -> ErrorOr<void> {
for (size_t i = 0; i < mounts.size(); ++i) {
auto& mount = mounts[i];
if (&mount->guest() != &guest_inode)
continue;
auto mountpoint_path = TRY(mount->absolute_path());
if (custody_path->view() != mountpoint_path->view())
continue;
TRY(mount->guest_fs().prepare_to_unmount());
mount->guest_fs().mounted_count({}).with([&](auto& mounted_count) {
mounted_count--;
});
dbgln("VirtualFileSystem: Unmounting file system {}...", mount->guest_fs().fsid());
(void)mounts.unstable_take(i);
return {};
@ -142,9 +155,13 @@ ErrorOr<void> VirtualFileSystem::mount_root(FileSystem& fs)
m_root_inode = root_inode;
auto pseudo_path = TRY(static_cast<FileBackedFileSystem&>(fs).file_description().pseudo_path());
dmesgln("VirtualFileSystem: mounted root from {} ({})", fs.class_name(), pseudo_path);
dmesgln("VirtualFileSystem: mounted root({}) from {} ({})", fs.fsid(), fs.class_name(), pseudo_path);
// Note: Actually add a mount for the filesystem and increment the filesystem mounted count
m_mounts.with([&](auto& mounts) {
new_mount->guest_fs().mounted_count({}).with([&](auto& mounted_count) {
mounted_count++;
});
mounts.append(move(new_mount));
});
@ -241,6 +258,24 @@ ErrorOr<InodeMetadata> VirtualFileSystem::lookup_metadata(Credentials const& cre
return custody->inode().metadata();
}
ErrorOr<NonnullLockRefPtr<FileBackedFileSystem>> VirtualFileSystem::find_already_existing_or_create_file_backed_file_system(OpenFileDescription& description, Function<ErrorOr<NonnullLockRefPtr<FileSystem>>(OpenFileDescription&)> callback)
{
return TRY(m_file_backed_file_systems_list.with([&](auto& list) -> ErrorOr<NonnullLockRefPtr<FileBackedFileSystem>> {
for (auto& node : list) {
if (&node.file_description() == &description) {
return node;
}
if (&node.file() == &description.file()) {
return node;
}
}
auto fs = TRY(callback(description));
VERIFY(fs->is_file_backed());
list.append(static_cast<FileBackedFileSystem&>(*fs));
return static_ptr_cast<FileBackedFileSystem>(fs);
}));
}
ErrorOr<NonnullLockRefPtr<OpenFileDescription>> VirtualFileSystem::open(Credentials const& credentials, StringView path, int options, mode_t mode, Custody& base, Optional<UidAndGid> owner)
{
if ((options & O_CREAT) && (options & O_DIRECTORY))