linux-kernel - Re: [PATCH v8 2/4] uprobe: use original page when all uprobes are removed

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Date:   Fri, 26 Jul 2019 10:44:23 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     lkml <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>
Subject: Re: [PATCH v8 2/4] uprobe: use original page when all uprobes are
 removed

On 07/25, Song Liu wrote:
>
> I guess I know the case now. We can probably avoid this with an simp.le 
> check for old_page == new_page?

better yet, I think we can check PageAnon(old_page) and avoid the unnecessary
__replace_page() in this case. See the patch below.

Anyway, why __replace_page() needs to lock both pages? This doesn't look nice
even if it were correct. I think it can do lock_page(old_page) later.

Oleg.


--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -488,6 +488,10 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 		ref_ctr_updated = 1;
 	}
 
+	ret = 0;
+	if (!is_register && !PageAnon(old_page))
+		goto put_old;
+
 	ret = anon_vma_prepare(vma);
 	if (ret)
 		goto put_old;