netdev - Re: [PATCH v8 1/7] net: introduce helper sendpage

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Date:   Sun, 27 Sep 2020 14:21:15 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Coly Li <colyli@...e.de>
Cc:     linux-block@...r.kernel.org, linux-nvme@...ts.infradead.org,
        netdev@...r.kernel.org, open-iscsi@...glegroups.com,
        linux-scsi@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chaitanya Kulkarni <chaitanya.kulkarni@....com>,
        Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
        Jan Kara <jack@...e.com>, Jens Axboe <axboe@...nel.dk>,
        Mikhail Skorzhinskii <mskorzhinskiy@...arflare.com>,
        Philipp Reisner <philipp.reisner@...bit.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Vlastimil Babka <vbabka@...e.com>, stable@...r.kernel.org
Subject: Re: [PATCH v8 1/7] net: introduce helper sendpage_ok() in
 include/linux/net.h

On Sat, Sep 26, 2020 at 09:28:03PM +0800, Coly Li wrote:
> On 2020/9/25 23:18, Greg KH wrote:
> > On Fri, Sep 25, 2020 at 11:01:13PM +0800, Coly Li 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.
> >>
> >> Signed-off-by: Coly Li <colyli@...e.de>
> >> Cc: Chaitanya Kulkarni <chaitanya.kulkarni@....com>
> >> Cc: Christoph Hellwig <hch@....de>
> >> Cc: Hannes Reinecke <hare@...e.de>
> >> Cc: Jan Kara <jack@...e.com>
> >> Cc: Jens Axboe <axboe@...nel.dk>
> >> Cc: Mikhail Skorzhinskii <mskorzhinskiy@...arflare.com>
> >> Cc: Philipp Reisner <philipp.reisner@...bit.com>
> >> Cc: Sagi Grimberg <sagi@...mberg.me>
> >> Cc: Vlastimil Babka <vbabka@...e.com>
> >> Cc: stable@...r.kernel.org
> >> ---
> >>  include/linux/net.h | 16 ++++++++++++++++
> >>  1 file changed, 16 insertions(+)
> >>
> >> diff --git a/include/linux/net.h b/include/linux/net.h
> >> index d48ff1180879..05db8690f67e 100644
> >> --- a/include/linux/net.h
> >> +++ b/include/linux/net.h
> >> @@ -21,6 +21,7 @@
> >>  #include <linux/rcupdate.h>
> >>  #include <linux/once.h>
> >>  #include <linux/fs.h>
> >> +#include <linux/mm.h>
> >>  #include <linux/sockptr.h>
> >>  
> >>  #include <uapi/linux/net.h>
> >> @@ -286,6 +287,21 @@ do {									\
> >>  #define net_get_random_once_wait(buf, nbytes)			\
> >>  	get_random_once_wait((buf), (nbytes))
> >>  
> >> +/*
> >> + * E.g. XFS meta- & log-data is in slab pages, or bcache meta
> >> + * data pages, or other high order pages allocated by
> >> + * __get_free_pages() without __GFP_COMP, which have a page_count
> >> + * of 0 and/or have PageSlab() set. We cannot use send_page for
> >> + * those, as that does get_page(); put_page(); and would cause
> >> + * either a VM_BUG directly, or __page_cache_release a page that
> >> + * would actually still be referenced by someone, leading to some
> >> + * obscure delayed Oops somewhere else.
> >> + */
> >> +static inline bool sendpage_ok(struct page *page)
> >> +{
> >> +	return  !PageSlab(page) && page_count(page) >= 1;
> > 
> > Do you have one extra ' ' after "return" there?
> 
> It should be fixed in next version.
> 
> > 
> > And this feels like a mm thing, why put it in net.h and not mm.h?
> 
> This check is specific for kernel_sendpage(), so I want to place it
> closer to where kernel_sendpage() is declared.
> 
> And indeed there was similar discussion about why this helper is not in
> mm code in v5 series. Christoph supported to place sendpage_ok() in
> net.h, an uncompleted piece of his opinion was "It is not a mm bug, it
> is a networking quirk."

Ah, nevermind then, sorry for the noise :)

greg k-h