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From:   Cong Wang <xiyou.wangcong@gmail.com>
Date:   Sun, 16 Aug 2020 10:55:09 -0700
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Subject: Re: [PATCH v5 1/3] net: introduce helper sendpage_ok() in include/linux/net.h
To:     Coly Li <colyli@suse.de>
Cc:     linux-block@vger.kernel.org, linux-nvme@lists.infradead.org,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
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        LKML <linux-kernel@vger.kernel.org>,
        Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>,
        Christoph Hellwig <hch@lst.de>, Hannes Reinecke <hare@suse.de>,
        Jan Kara <jack@suse.com>, Jens Axboe <axboe@kernel.dk>,
        Mikhail Skorzhinskii <mskorzhinskiy@solarflare.com>,
        Philipp Reisner <philipp.reisner@linbit.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Vlastimil Babka <vbabka@suse.com>
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On Sun, Aug 16, 2020 at 1:36 AM Coly Li <colyli@suse.de> wrote:
>
> The original problem was from nvme-over-tcp code, who mistakenly uses
> kernel_sendpage() to send pages allocated by __get_free_pages() without
> __GFP_COMP flag. Such pages don't have refcount (page_count is 0) on
> tail pages, sending them by kernel_sendpage() may trigger a kernel panic
> from a corrupted kernel heap, because these pages are incorrectly freed
> in network stack as page_count 0 pages.
>
> This patch introduces a helper sendpage_ok(), it returns true if the
> checking page,
> - is not slab page: PageSlab(page) is false.
> - has page refcount: page_count(page) is not zero
>
> All drivers who want to send page to remote end by kernel_sendpage()
> may use this helper to check whether the page is OK. If the helper does
> not return true, the driver should try other non sendpage method (e.g.
> sock_no_sendpage()) to handle the page.

Can we leave this helper to mm subsystem?

I know it is for sendpage, but its implementation is all about some
mm details and its two callers do not belong to net subsystem either.

Think this in another way: who would fix it if it is buggy? I bet mm people
should. ;)

Thanks.