Kernel/FATFS: Keep the FSInfo sector's free cluster count in sync

Author: https://github.com/implicitfield
Commit: 32692f032c
Pull-request: https://github.com/SerenityOS/serenity/pull/23907
Reviewed-by: https://github.com/Hendiadyoin1
Reviewed-by: https://github.com/cqundefine
Reviewed-by: https://github.com/nico
Reviewed-by: https://github.com/timschumi ✅

Kernel/FileSystem/FATFS/FileSystem.cpp

@@ -371,6 +371,17 @@ u32 FATFS::end_of_chain_marker() const
     }
 }
 
+ErrorOr<void> FATFS::set_free_cluster_count(u32 value)
+{
+    VERIFY(m_fat_version == FATVersion::FAT32);
+
+    m_fs_info.last_known_free_cluster_count = value;
+    auto fs_info_buffer = UserOrKernelBuffer::for_kernel_buffer(bit_cast<u8*>(&m_fs_info));
+    TRY(write_block(m_parameter_block->dos7_bpb()->fs_info_sector, fs_info_buffer, sizeof(m_fs_info)));
+
+    return {};
+}
+
 ErrorOr<u32> FATFS::allocate_cluster()
 {
     u32 start_cluster;
@@ -393,6 +404,10 @@ ErrorOr<u32> FATFS::allocate_cluster()
     for (u32 i = start_cluster; i < m_parameter_block->sector_count() / m_parameter_block->common_bpb()->sectors_per_cluster; i++) {
         if (TRY(fat_read(i)) == 0) {
             dbgln_if(FAT_DEBUG, "FATFS: Allocating cluster {}", i);
+
+            if (m_fat_version == FATVersion::FAT32 && m_fs_info.last_known_free_cluster_count != fs_info_data_unknown)
+                TRY(set_free_cluster_count(m_fs_info.last_known_free_cluster_count - 1));
+
             TRY(fat_write(i, end_of_chain_marker()));
             return i;
         }
@@ -401,6 +416,14 @@ ErrorOr<u32> FATFS::allocate_cluster()
     return Error::from_errno(ENOSPC);
 }
 
+ErrorOr<void> FATFS::notify_cluster_freed()
+{
+    if (m_fat_version == FATVersion::FAT32 && m_fs_info.last_known_free_cluster_count != fs_info_data_unknown)
+        TRY(set_free_cluster_count(m_fs_info.last_known_free_cluster_count + 1));
+
+    return {};
+}
+
 ErrorOr<u32> FATFS::fat_read(u32 cluster)
 {
     dbgln_if(FAT_DEBUG, "FATFS: Reading FAT entry for cluster {}", cluster);

Kernel/FileSystem/FATFS/FileSystem.h

@@ -87,7 +87,10 @@ private:
     static constexpr u32 first_data_cluster = 2;
 
     FatBlockSpan first_block_of_cluster(u32 cluster) const;
+
+    ErrorOr<void> set_free_cluster_count(u32);
     ErrorOr<u32> allocate_cluster();
+    ErrorOr<void> notify_cluster_freed();
 
     size_t fat_offset_for_cluster(u32 cluster) const;
 

Kernel/FileSystem/FATFS/Inode.cpp

@@ -346,6 +346,7 @@ ErrorOr<void> FATInode::remove_last_cluster_from_chain()
 
     dbgln_if(FAT_DEBUG, "FATInode[{}]::remove_last_cluster_from_chain(): freeing cluster {}", identifier(), last_cluster);
 
+    TRY(fs().notify_cluster_freed());
     m_cluster_list.remove(m_cluster_list.size() - 1);
 
     if (m_cluster_list.is_empty() || (m_cluster_list.size() == 1 && first_cluster() <= 1)) {