Re: [PATCH v3 1/3] mm: rename alloc_pages_exact_node to __alloc_pages_node

On Thu, Jul 30, 2015 at 06:34:29PM +0200, Vlastimil Babka wrote:

The function alloc_pages_exact_node() was introduced in 6484eb3e2a81 ("page
allocator: do not check NUMA node ID when the caller knows the node is valid")
as an optimized variant of alloc_pages_node(), that doesn't fallback to current
node for nid == NUMA_NO_NODE. Unfortunately the name of the function can easily
suggest that the allocation is restricted to the given node and fails
otherwise. In truth, the node is only preferred, unless __GFP_THISNODE is
passed among the gfp flags.

The misleading name has lead to mistakes in the past, see 5265047ac301 ("mm,
thp: really limit transparent hugepage allocation to local node") and
b360edb43f8e ("mm, mempolicy: migrate_to_node should only migrate to node").

Another issue with the name is that there's a family of alloc_pages_exact*()
functions where 'exact' means exact size (instead of page order), which leads
to more confusion.

To prevent further mistakes, this patch effectively renames
alloc_pages_exact_node() to __alloc_pages_node() to better convey that it's
an optimized variant of alloc_pages_node() not intended for general usage.
Both functions get described in comments.

It has been also considered to really provide a convenience function for
allocations restricted to a node, but the major opinion seems to be that
__GFP_THISNODE already provides that functionality and we shouldn't duplicate
the API needlessly. The number of users would be small anyway.

Existing callers of alloc_pages_exact_node() are simply converted to call
__alloc_pages_node(), with two exceptions. sba_alloc_coherent() and
slob_new_page() both open-code the check for NUMA_NO_NODE, so they are
converted to use alloc_pages_node() instead. This means they no longer perform
some VM_BUG_ON checks, and since the current check for nid in
alloc_pages_node() uses a 'nid < 0' comparison (which includes NUMA_NO_NODE),
it may hide wrong values which would be previously exposed. Both differences
will be rectified by the next patch.

To sum up, this patch makes no functional changes, except temporarily hiding
potentially buggy callers. Restricting the checks in alloc_pages_node() is
left for the next patch which can in turn expose more existing buggy callers.

Signed-off-by: Vlastimil Babka <redacted>
Cc: Mel Gorman <mgorman@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Greg Thelen <redacted>
Cc: Aneesh Kumar K.V <redacted>
Cc: Christoph Lameter <redacted>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <redacted>
Cc: Naoya Horiguchi <redacted>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <redacted>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Cliff Whickman <redacted>
Acked-by: Robin Holt <robinmholt@gmail.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>