mm: readahead: get back a sensible upper limit

mm: readahead: get back a sensible upper limit

commit 6d2be915e589 ("mm/readahead.c: fix readahead failure for memoryless NUMA
nodes and limit readahead pages")[1] imposed 2 mB hard limits to readahead by
changing max_sane_readahead() to sort out a corner case where a thread runs on
amemoryless NUMA node and it would have its readahead capability disabled.

The aforementioned change, despite fixing that corner case, is detrimental to
other ordinary workloads that memory map big files and rely on readahead() or
posix_fadvise(WILLNEED) syscalls to get most of the file populating system's cache.

Laurence Oberman reports, via https://bugzilla.redhat.com/show_bug.cgi?id=1187940,
slowdowns up to 3-4 times when changes for mentioned commit [1] got introduced in
RHEL kenrel. We also have an upstream bugzilla opened for similar complaint:
https://bugzilla.kernel.org/show_bug.cgi?id=79111

This patch brings back the old behavior of max_sane_readahead() where we used to
consider NR_INACTIVE_FILE and NR_FREE_PAGES pages to derive a sensible / adujstable
readahead upper limit. This patch also keeps the 2 mB ceiling scheme introduced by
commit [1] to avoid regressions on CONFIG_HAVE_MEMORYLESS_NODES systems,
where numa_mem_id(), by any buggy reason, might end up not returning
the 'local memory' for a memoryless node CPU.

Reported-by: Laurence Oberman <loberman@redhat.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Rafael Aquini <aquini@redhat.com>