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Subject[PATCH 13/14] readahead: enforce full readahead size on async mmap readahead
We need this in one perticular 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: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/filemap.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- mm.orig/mm/filemap.c
+++ mm/mm/filemap.c
@@ -1584,7 +1584,8 @@ static void do_async_mmap_readahead(stru
if (ra->mmap_miss > 0)
ra->mmap_miss--;
if (PageReadahead(page))
- page_cache_async_readahead(mapping, ra, file, page, offset, 1);
+ page_cache_async_readahead(mapping, ra, file,
+ page, offset, ra->ra_pages);
}

/**
--



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