• W
    readahead: enforce full readahead size on async mmap readahead · 2fad6f5d
    Wu Fengguang 提交于
    We need this in one particular case and two more general ones.
    
    Now we do async readahead for sequential mmap reads, and do it with the
    help of PG_readahead.  For normal reads, PG_readahead is the sufficient
    condition to do a sequential readahead.  But unfortunately, for mmap
    reads, there is a tiny nuisance:
    
    [11736.998347] readahead-init0(process: sh/23926, file: sda1/w3m, offset=0:4503599627370495, ra=0+4-3) = 4
    [11737.014985] readahead-around(process: w3m/23926, file: sda1/w3m, offset=0:0, ra=290+32-0) = 17
    [11737.019488] readahead-around(process: w3m/23926, file: sda1/w3m, offset=0:0, ra=118+32-0) = 32
    [11737.024921] readahead-interleaved(process: w3m/23926, file: sda1/w3m, offset=0:2, ra=4+6-6) = 6
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                 ~~~~~~~~~~~~~
    
    An unfavorably small readahead.  The original dumb read-around size could
    be more efficient.
    
    That happened because ld-linux.so does a read(832) in L1 before mmap(),
    which triggers a 4-page readahead, with the second page tagged
    PG_readahead.
    
    L0: open("/lib/libc.so.6", O_RDONLY)        = 3
    L1: read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\342"..., 832) = 832
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    L2: fstat(3, {st_mode=S_IFREG|0755, st_size=1420624, ...}) = 0
    L3: mmap(NULL, 3527256, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fac6e51d000
    L4: mprotect(0x7fac6e671000, 2097152, PROT_NONE) = 0
    L5: mmap(0x7fac6e871000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x154000) = 0x7fac6e871000
    L6: mmap(0x7fac6e876000, 16984, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fac6e876000
    L7: close(3)                                = 0
    
    In general, the PG_readahead flag will also be hit in cases
    
    - sequential reads
    
    - clustered random reads
    
    A full readahead size is desirable in both cases.
    
    Cc: Nick Piggin <npiggin@suse.de>
    Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>
    Cc: Ying Han <yinghan@google.com>
    Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
    2fad6f5d
filemap.c 66.4 KB