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

John Hubbard
2019-08-05T22:54:40Z

On 7/23/19 11:17 PM, Christoph Hellwig wrote:
> On Tue, Jul 23, 2019 at 09:25:06PM -0700, john.hubbard@gmail.com 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 іn 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.
>