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

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Message-ID: <20190724113711.GE21599@redhat.com>
Date:   Wed, 24 Jul 2019 13:37:11 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, matthew.wilcox@...cle.com,
        kirill.shutemov@...ux.intel.com, peterz@...radead.org,
        rostedt@...dmis.org, kernel-team@...com,
        william.kucharski@...cle.com
Subject: Re: [PATCH v8 2/4] uprobe: use original page when all uprobes are
 removed

On 07/24, Song Liu wrote:
>
>  	lock_page(old_page);
> @@ -177,15 +180,24 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
>  	mmu_notifier_invalidate_range_start(&range);
>  	err = -EAGAIN;
>  	if (!page_vma_mapped_walk(&pvmw)) {
> -		mem_cgroup_cancel_charge(new_page, memcg, false);
> +		if (!orig)
> +			mem_cgroup_cancel_charge(new_page, memcg, false);
>  		goto unlock;
>  	}
>  	VM_BUG_ON_PAGE(addr != pvmw.address, old_page);
>  
>  	get_page(new_page);
> -	page_add_new_anon_rmap(new_page, vma, addr, false);
> -	mem_cgroup_commit_charge(new_page, memcg, false, false);
> -	lru_cache_add_active_or_unevictable(new_page, vma);
> +	if (orig) {
> +		lock_page(new_page);  /* for page_add_file_rmap() */
> +		page_add_file_rmap(new_page, false);


Shouldn't we re-check new_page->mapping after lock_page() ? Or we can't
race with truncate?


and I am worried this code can try to lock the same page twice...
Say, the probed application does MADV_DONTNEED and then writes "int3"
into vma->vm_file at the same address to fool verify_opcode().

Oleg.