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