Re: NFS directio

[Chuck Lever <cel@citi.umich.edu>]

Neil Brown wrote:
> On Friday March 31, cel@citi.umich.edu wrote:
>> Olaf Kirch wrote:
>>> On Fri, Mar 31, 2006 at 09:35:34AM -0500, Chuck Lever wrote:
>>>> the check isn't in 2.6.16.  it was removed sometime after 2.6.5.
>>> It is still in the 2.6.16 tree I'm looking at; else I wouldn't ask :)
>> it's been in my trees since 2.6.13 or even earlier, my mistake.
>>
>> that change is part of the aio+dio patches that were just included in 
>> 2.6.17-rc1.  instead of creating a single patch for this change, you 
>> should consider taking those patches, since they were tested as a unit.
>>
>> if you can guarantee that atomic_t is 32-bits on every platform you 
>> support, then it should be save to change that #define to 2^31. 
>> otherwise, the work to eliminate the limit entirely has already been 
>> done by the above-mentioned patches.
> 
> (Coming into the conversation a bit late....)
> 
> What about the kmalloc in nfs_get_user_pages:
> 
> 	array_size = (page_count * sizeof(struct page *));
> 	*pages = kmalloc(array_size, GFP_KERNEL);
> 
> With a page_count of 1024, this allocates one page (on 32bit) which is
> easy.
> With a page_count of 4096 (the previous MAX_DIRECTIO_SIZE)), this
> allocates 4 consecutive pages, which won't always succeed.
> 
> If you want to go higher than that (which was the point of the start
> of this thread) then you need a large-order allocation which doesn't
> (in my understanding) have a good chance of success due to
> fragmentation.
> 
> So I guess my question is: how hard would it be to use a more scalable
> data structure so that very large IO sizes would be reliably
> practical?

howdy neil-

usually I/O is broken up into smaller chunks by the time it gets down to 
this level, so it's never been much of an issue.  it's pretty 
challenging to generate a test case for extremely large I/O sizes (for 
example, the size of the entire process address space).

and until now, there really hasn't been much call for doing NFS O_DIRECT 
with very large requests.  it's been a matter of meeting the 
requirements of database I/O, which is generally 4KB to 16KB for data 
files, and about a megabyte for log writes.

at this point we don't really have a test case and a use case that 
reliably breaks this, so it hasn't been a priority to address this.

the structure of this code was adapted (ie stolen) from other parts of 
the kernel that also employ get_user_pages.  you can probably take a 
look at other places that employ get_user_pages(), and see how they've 
since tackled the issue.

-- 
corporate:	<cel at netapp dot com>
personal:	<chucklever at bigfoot dot com>


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