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Date: Mon, 13 Jun 2011 15:46:36 +0200
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v4 3.0-rc2-tip 2/22]  2: uprobes: Breakground page
	replacement.
Message-ID: <20110613134636.GA21979@redhat.com>
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To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>, Linux-mm <linux-mm@kvack.org>, Arnaldo Carvalho de Melo <acme@infradead.org>, Linus Torvalds <torvalds@linux-foundation.org>, Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>, Hugh Dickins <hughd@google.com>, Christoph Hellwig <hch@infradead.org>, Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>, Jonathan Corbet <corbet@lwn.net>, Andrew Morton <akpm@linux-foundation.org>, Jim Keniston <jkenisto@linux.vnet.ibm.com>, Roland McGrath <roland@hack.frob.com>, Ananth N Mavinakayanahalli <ananth@in.ibm.com>, LKML <linux-kernel@vger.kernel.org>

On 06/07, Srikar Dronamraju wrote:
>
> +static int __replace_page(struct vm_area_struct *vma, struct page *page,
> +					struct page *kpage)
> +{
> +	struct mm_struct *mm = vma->vm_mm;
> +	pgd_t *pgd;
> +	pud_t *pud;
> +	pmd_t *pmd;
> +	pte_t *ptep;
> +	spinlock_t *ptl;
> +	unsigned long addr;
> +	int err = -EFAULT;
> +
> +	addr = page_address_in_vma(page, vma);
> +	if (addr == -EFAULT)
> +		goto out;
> +
> +	pgd = pgd_offset(mm, addr);
> +	if (!pgd_present(*pgd))
> +		goto out;
> +
> +	pud = pud_offset(pgd, addr);
> +	if (!pud_present(*pud))
> +		goto out;
> +
> +	pmd = pmd_offset(pud, addr);
> +	if (pmd_trans_huge(*pmd) || (!pmd_present(*pmd)))
> +		goto out;

Hmm. So it doesn't work with transhuge pages? May be the caller should
use __gup(FOLL_SPLIT), otherwise set_bkpt/etc can fail "mysteriously", no?
OTOH, I don't really understand how pmd_trans_huge() is possible, valid_vma()
checks ->vm_file != NULL and I iiuc transparent hugepages can only work
with anonymous mappings. Confused...

But the real problem (afaics) is VM_HUGETLB mappings. I can't understand
how __replace_page() can work in this case. Probably valid_vma() should
fail if is_vm_hugetlb_page()?

Oleg.

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