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

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:

quoted

* 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.

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

On Mon, Aug 05, 2019 at 03:54:35PM -0700, John Hubbard wrote:

On 7/23/19 11:17 PM, Christoph Hellwig wrote:

quoted

On Tue, Jul 23, 2019 at 09:25:06PM -0700, john.hubbard@gmail.com wrote:

quoted

* 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?

I was hoping I could steer you towards it.  But if you don't want to do
it yourself I'll add it to my ever growing todo list.

Or on the put_user_bvec() idea?

I have a prototype from two month ago:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec

but that only survived the most basic testing, so it'll need more work,
which I'm not sure when I'll find time for.
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization