tree eca8262062c4fda63cb3bd34f9478f3fbfd7f518
parent 70ac23cfa31f68289d4b720c6162b3929ab4de36
author Wu Fengguang <fengguang.wu@intel.com> 1245191489 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> 1245206849 -0700

readahead: enforce full readahead size on async mmap readahead

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: Wu Fengguang <fengguang.wu@intel.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>