Re: allowed pages in the block later, was Re: [Ext2-devel] [PATCH] ext3: avoid sending down non-refcounted pages

[Mike Christie <michaelc@cs.wisc.edu>]

Mike Christie wrote:
> 
> Christoph Hellwig wrote:
> 
>> On Thu, Dec 08, 2005 at 03:18:33AM -0700, Andreas Dilger wrote:
>>
>>> What happens on 1kB or 2kB block filesystems (i.e. b_size != PAGE_SIZE)?
>>> This will allocate a whole page for each block (which may be 
>>> considerable
>>> overhead on e.g. a 64kB PAGE_SIZE ia64 or PPC system).
>>
>>
>>
>> Yes.  How often do we trigger this codepath?
>>
>> The problem we're trying to solve here is how do implement network block
>> devices (nbd, iscsi) efficiently.  The zero copy codepath in the 
>> networking
>> layer does need to grab additional references to pages.  So to use 
>> sendpage
>> we need a refcountable page.  pages used by the slab allocator are not
>> normally refcounted so try to do get_page/pub_page on them will break.
>>
>> One way to work around that would be to detect kmalloced pages and use
>> a slowpath for that.  The major issues with that is that we don't have a
>> reliable way to detect if a given struct page comes from the slab 
>> allocator
>> or not.  The minor problem is that even with such an indicator it means
>> having a separate and lightly tested slowpath for this rare case.
>>
>> All in all I think we should document that the block layer only accepts
>> properly refcounted pages, which is everything but kmalloced pages (even
>> vmalloc is totally fine)
> 
> 
> Is it anytime kmalloc is used? For scsi when it uses scsi_execute* for 
> something like scanning (report luns result is kmallocd) would this be a 
> problem?
> 
> If PageSlab() does work, then could we have a request queue flag that 
> bounces those pages for all block layer drivers. Pretty slow and yucky 
> but if we have to convert SCSI and maybe other parts of the block layer 
> maybe it will be easiest for now.
> 

Or there is not a way to do kmalloc(GFP_BLK) that gives us the right 
type of memory is there?