Re: [PATCH 00/12] block/bio, fs: convert put_page() to put_user_page*()

[John Hubbard]

On 7/23/19 11:17 PM, Christoph Hellwig wrote:
> On Tue, Jul 23, 2019 at 09:25:06PM -0700, john.hubbard@xxxxxxxxx wrote:
>> * Store, in the iov_iter, a "came from gup (get_user_pages)" parameter.
>>   Then, use the new iov_iter_get_pages_use_gup() to retrieve it when
>>   it is time to release the pages. That allows choosing between put_page()
>>   and put_user_page*().
>>
>> * Pass in one more piece of information to bio_release_pages: a "from_gup"
>>   parameter. Similar use as above.
>>
>> * Change the block layer, and several file systems, to use
>>   put_user_page*().
> 
> I think we can do this in a simple and better way.  We have 5 ITER_*
> types.  Of those ITER_DISCARD as the name suggests never uses pages, so
> we can skip handling it.  ITER_PIPE is rejected Ñn the direct I/O path,
> which leaves us with three.
> 

Hi Christoph,

Are you working on anything like this? Or on the put_user_bvec() idea?
Please let me know, otherwise I'll go in and implement something here.


thanks,
-- 
John Hubbard
NVIDIA

> Out of those ITER_BVEC needs a user page reference, so we want to call
> put_user_page* on it.  ITER_BVEC always already has page reference,
> which means in the block direct I/O path path we alread don't take
> a page reference.  We should extent that handling to all other calls
> of iov_iter_get_pages / iov_iter_get_pages_alloc.  I think we should
> just reject ITER_KVEC for direct I/O as well as we have no users and
> it is rather pointless.  Alternatively if we see a use for it the
> callers should always have a life page reference anyway (or might
> be on kmalloc memory), so we really should not take a reference either.
> 
> In other words:  the only time we should ever have to put a page in
> this patch is when they are user pages.  We'll need to clean up
> various bits of code for that, but that can be done gradually before
> even getting to the actual put_user_pages conversion.
>