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Subject: Re: [PATCH RFC 0/2] mm/page_alloc: free_pcppages_bulk safeguard
To:     Mel Gorman <mgorman@techsingularity.net>,
        Chris Li <chrisl@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        baolin.wang@linux.alibaba.com, Michal Hocko <mhocko@suse.com>,
        david@redhat.com, willy@infradead.org, linux-mm@kvack.org,
        Namhyung Kim <namhyung@google.com>,
        Greg Thelen <gthelen@google.com>, linux-kernel@vger.kernel.org,
        John Sperbeck <jsperbeck@google.com>
References: <20230817-free_pcppages_bulk-v1-0-c14574a9f80c@kernel.org>
 <20230821103225.qntnsotdzuthxn2y@techsingularity.net>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <6a18b295-92ac-1633-0b79-6aaf18a8496d@huaweicloud.com>
Date:   Tue, 22 Aug 2023 09:27:42 +0800
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on 8/21/2023 6:32 PM, Mel Gorman wrote:
> On Thu, Aug 17, 2023 at 11:05:22PM -0700, Chris Li wrote:
>> In this patch series I want to safeguard
>> the free_pcppage_bulk against change in the
>> pcp->count outside of this function. e.g.
>> by BPF program inject on the function tracepoint.
>>
>> I break up the patches into two seperate patches
>> for the safeguard and clean up.
>>
>> Hopefully that is easier to review.
>>
>> Signed-off-by: Chris Li <chrisl@kernel.org>
> 
> This sounds like a maintenance nightmare if internal state can be arbitrary
> modified by a BPF program and still expected to work properly in all cases.
> Every review would have to take into account "what if a BPF script modifies
> state behind our back?"
> 
Agreed. We assume pcp->count is protected by pcp->lock. Instead of make code
work in case pcp->count could be changed without lock held, it's more reasonble
to modify pcp->count with pcp->lock held in BPF program.