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To save more space (equivalent to the size of one erase sector of MTD device) and to achieve faster read and write speeds, a method for direct writing was introduced at the FTL layer. This can be accomplished simply by using the following oflags during the open operation: 1. O_DIRECT. when this flag is passed in, ftl internally uses the direct write strategy and no read cache is used in ftl; otherwise, each write will be executed with the minimum granularity of flash erase sector size which means a "sector read back - erase sector - write sector" operation is performed by using a read cache buffer in heap. 2. O_SYNC. When this flag is passed in, we assume that the flash has been erased in advance and no erasure operation will be performed internally within ftl. O_SYNC will take effect only when both O_DIRECT and O_SYNC are passed in simultaneously. 3. For uniformity, we remapped the mount flag in mount.h and unified it with the open flag in fcntl.h. The repetitive parts of their definitions were reused, and the remaining part of the mount flag redefine to the unused bit of open flags. Signed-off-by: jingfei <jingfei@xiaomi.com>