linux-kernel - Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page

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Date:	Sun, 27 Sep 2009 17:26:25 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Nick Piggin <npiggin@...e.de>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()

On Sat, 26 Sep 2009, Andi Kleen wrote:
> > This is a bit tricky to do right now; you have a chicken and egg
> > problem between locking the page and pinning the inode mapping.
> 
> One possibly simple solution would be to just allocate the page
> locked (GFP_LOCKED). When the allocator clears the flags it already
> modifies the state, so it could as well set the lock bit too. No
> atomics needed.  And then clearing it later is also atomic free.

That's a good idea.

I don't particularly like adding a GFP_LOCKED just for this, and I
don't particularly like having to remember to unlock the thing on the
various(?) error paths between getting the page and adding it to cache.

But it is a good idea, and if doing it that way would really close a
race window which checking page->mapping (or whatever) cannot (I'm
simply not sure about that), then it would seem the best way to go.

Hugh
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