linux-kernel - Re: [PATCH] spinlock in function hugetlb

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Message-ID: <a0a62dfc0707230727r3d9b5f18i69156073e6e6d4a5@mail.gmail.com>
Date:	Mon, 23 Jul 2007 09:27:06 -0500
From:	"Adam Litke" <aglitke@...il.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, agl@...ibm.com
Subject: Re: [PATCH] spinlock in function hugetlb_fault could be deleted

Hello.  hugetlb_instantiation_mutex is an extremely heavy-weight lock
whose days are numbered (hopefully).  It exists primarily to arbitrate
a race condition where n (n > 1) threads of execution race to satisfy
the same page fault for a process.  Even though only one hugetlb page
is needed, if (n) are not available, the application can receive a
bogus VM_FAULT_OOM.

Anyway, the hugetlb_instantiation_mutex approach has few friends
around here, so rather than making the code rely more heavily upon it,
perhaps you could focus you efforts on helping us remove it.

On 7/23/07, Zhang, Yanmin <yanmin_zhang@...ux.intel.com> wrote:
> Function hugetlb_fault needn't hold spinlock mm->page_table_lock,
> because when hugetlb_fault is called:
> 1) mm->mmap_sem is held already;
> 2) hugetlb_instantiation_mutex is held by hugetlb_fault, which prevents
> other threads/processes from entering this critical area. It's impossible
> for other threads/processes to change the page table now.

--
Adam Litke ( agl at us.ibm.com )
IBM Linux Technology Center
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