Re: [PATCH v8 1/7] net: introduce helper sendpage_ok() in include/linux/net.h

[Coly Li]

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@xxxxxxx>
>> Cc: Chaitanya Kulkarni <chaitanya.kulkarni@xxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxx>
>> Cc: Hannes Reinecke <hare@xxxxxxx>
>> Cc: Jan Kara <jack@xxxxxxxx>
>> Cc: Jens Axboe <axboe@xxxxxxxxx>
>> Cc: Mikhail Skorzhinskii <mskorzhinskiy@xxxxxxxxxxxxxx>
>> Cc: Philipp Reisner <philipp.reisner@xxxxxxxxxx>
>> Cc: Sagi Grimberg <sagi@xxxxxxxxxxx>
>> Cc: Vlastimil Babka <vbabka@xxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> ---
>>  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."

Thanks.

Coly Li