netdev - Re: [RFC 3/4] sparc: remove __GFP

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Message-ID: <20150302203304.GA20513@dhcp22.suse.cz>
Date:	Mon, 2 Mar 2015 21:33:04 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	David Miller <davem@...emloft.net>
Cc:	linux-mm@...ck.org, akpm@...ux-foundation.org, hannes@...xchg.org,
	rientjes@...gle.com, david@...morbit.com, tytso@....edu,
	mgorman@...e.de, penguin-kernel@...ove.SAKURA.ne.jp,
	sparclinux@...r.kernel.org, vipul@...lsio.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/4] sparc: remove __GFP_NOFAIL reuquirement

On Mon 02-03-15 15:04:05, David S. Miller wrote:
> From: Michal Hocko <mhocko@...e.cz>
> Date: Mon,  2 Mar 2015 14:54:42 +0100
> 
> > mdesc_kmalloc is currently requiring __GFP_NOFAIL allocation although it
> > seems that the allocation failure is handled by all callers (via
> > mdesc_alloc). __GFP_NOFAIL is a strong liability for the memory
> > allocator and so the users are discouraged to use the flag unless the
> > allocation failure is really a nogo. Drop the flag here as this doesn't
> > seem to be the case.
> > 
> > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> 
> It is a serious failure.
> 
> If we miss an MDESC update due to this allocation failure, the update
> is not an event which gets retransmitted so we will lose the updated
> machine description forever.
> 
> We really need this allocation to succeed.

OK, thanks for the clarification. This wasn't clear from the commit
which has introduced this code. I will drop this patch. Would you
accept something like the following instead?
---
diff --git a/arch/sparc/kernel/mdesc.c b/arch/sparc/kernel/mdesc.c
index 99632a87e697..26c80e18d7b1 100644
--- a/arch/sparc/kernel/mdesc.c
+++ b/arch/sparc/kernel/mdesc.c
@@ -130,26 +130,26 @@ static struct mdesc_mem_ops memblock_mdesc_ops = {
 static struct mdesc_handle *mdesc_kmalloc(unsigned int mdesc_size)
 {
 	unsigned int handle_size;
+	struct mdesc_handle *hp;
+	unsigned long addr;
 	void *base;
 
 	handle_size = (sizeof(struct mdesc_handle) -
 		       sizeof(struct mdesc_hdr) +
 		       mdesc_size);
 
+	/*
+	 * Allocation has to succeed because mdesc update would be missed
+	 * and such events are not retransmitted.
+	 */
 	base = kmalloc(handle_size + 15, GFP_KERNEL | __GFP_NOFAIL);
-	if (base) {
-		struct mdesc_handle *hp;
-		unsigned long addr;
-
-		addr = (unsigned long)base;
-		addr = (addr + 15UL) & ~15UL;
-		hp = (struct mdesc_handle *) addr;
+	addr = (unsigned long)base;
+	addr = (addr + 15UL) & ~15UL;
+	hp = (struct mdesc_handle *) addr;
 
-		mdesc_handle_init(hp, handle_size, base);
-		return hp;
-	}
+	mdesc_handle_init(hp, handle_size, base);
 
-	return NULL;
+	return hp;
 }
 
 static void mdesc_kfree(struct mdesc_handle *hp)

-- 
Michal Hocko
SUSE Labs
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