linux-kernel - Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement.

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Date:	Mon, 13 Jun 2011 15:46:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 2/22]  2: uprobes: Breakground page
	replacement.

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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