f2fs: add segment operations
This adds specific functions not only to manage dirty/free segments, SIT pages, a cache for SIT entries, and summary entries, but also to allocate free blocks and write three types of pages: data, node, and meta. - F2FS maintains three types of bitmaps in memory, which indicate free, prefree, and dirty segments respectively. - The key information of an SIT entry consists of a segment number, the number of valid blocks in the segment, a bitmap to identify there-in valid or invalid blocks. - An SIT page is composed of a certain range of SIT entries, which is maintained by the address space of meta_inode. - To cache SIT entries, a simple array is used. The index for the array is the segment number. - A summary entry for data contains the parent node information. A summary entry for node contains its node offset from the inode. - F2FS manages information about six active logs and those summary entries in memory. Whenever one of them is changed, its summary entries are flushed to its SIT page maintained by the address space of meta_inode. - This patch adds a default block allocation function which supports heap-based allocation policy. - This patch adds core functions to write data, node, and meta pages. Since LFS basically produces a series of sequential writes, F2FS merges sequential bios with a single one as much as possible to reduce the IO scheduling overhead. Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
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