linux-kernel - Re: [PATCH v3 1/3] mm: rename alloc_pages_exact_node to __alloc_pages

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Message-ID: <20150730173318.GA15257@cmpxchg.org>
Date:	Thu, 30 Jul 2015 13:33:18 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Greg Thelen <gthelen@...gle.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	cbe-oss-dev@...ts.ozlabs.org, kvm@...r.kernel.org,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Arnd Bergmann <arnd@...db.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Cliff Whickman <cpw@....com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Robin Holt <robinmholt@...il.com>
Subject: 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 <vbabka@...e.cz>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Greg Thelen <gthelen@...gle.com>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Pekka Enberg <penberg@...nel.org>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Fenghua Yu <fenghua.yu@...el.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Acked-by: Michael Ellerman <mpe@...erman.id.au>
> Cc: Gleb Natapov <gleb@...nel.org>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Cliff Whickman <cpw@....com>
> Acked-by: Robin Holt <robinmholt@...il.com>

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