fs/sync.c: sync_file_range(2) may use WB_SYNC_ALL writeback
23d01270 ("fs/sync.c: make sync_file_range(2) use WB_SYNC_NONE writeback") claims that sync_file_range(2) syscall was "created for userspace to be able to issue background writeout and so waiting for in-flight IO is undesirable there" and changes the writeback (back) to WB_SYNC_NONE. This claim is only partially true. It is true for users that use the flag SYNC_FILE_RANGE_WRITE by itself, as does PostgreSQL, the user that was the reason for changing to WB_SYNC_NONE writeback. However, that claim is not true for users that use that flag combination SYNC_FILE_RANGE_{WAIT_BEFORE|WRITE|_WAIT_AFTER}. Those users explicitly requested to wait for in-flight IO as well as to writeback of dirty pages. Re-brand that flag combination as SYNC_FILE_RANGE_WRITE_AND_WAIT and use WB_SYNC_ALL writeback to perform the full range sync request. Link: http://lkml.kernel.org/r/20190409114922.30095-1-amir73il@gmail.com Link: http://lkml.kernel.org/r/2019041907...
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