Re: [PATCH] mm/gup: Use put_user_page*() instead of put_page*()

[John Hubbard]

On 7/14/19 12:08 PM, Bharath Vedartham wrote:
> This patch converts all call sites of get_user_pages
> to use put_user_page*() instead of put_page*() functions to
> release reference to gup pinned pages.

Hi Bharath,

Thanks for jumping in to help, and welcome to the party!

You've caught everyone in the middle of a merge window, btw.  As a
result, I'm busy rebasing and reworking the get_user_pages call sites, 
and gup tracking, in the wake of some semi-traumatic changes to bio 
and gup and such. I plan to re-post right after 5.3-rc1 shows up, from 
here:

    https://github.com/johnhubbard/linux/commits/gup_dma_core

...which you'll find already covers the changes you've posted, except for:

    drivers/misc/sgi-gru/grufault.c
    drivers/staging/kpc2000/kpc_dma/fileops.c

...and this one, which is undergoing to larger local changes, due to
bvec, so let's leave it out of the choices:

    fs/io_uring.c

Therefore, until -rc1, if you'd like to help, I'd recommend one or more
of the following ideas:

1. Pull down https://github.com/johnhubbard/linux/commits/gup_dma_core
and find missing conversions: look for any additional missing 
get_user_pages/put_page conversions. You've already found a couple missing 
ones. I haven't re-run a search in a long time, so there's probably even more.

	a) And find more, after I rebase to 5.3-rc1: people probably are adding
	get_user_pages() calls as we speak. :)

2. Patches: Focus on just one subsystem at a time, and perfect the patch for
it. For example, I think this the staging driver would be perfect to start with:

    drivers/staging/kpc2000/kpc_dma/fileops.c

	a) verify that you've really, corrected converted the whole
	driver. (Hint: I think you might be overlooking a put_page call.)

	b) Attempt to test it if you can (I'm being hypocritical in
	the extreme here, but one of my problems is that testing
	has been light, so any help is very valuable). qemu...?
	OTOH, maybe even qemu cannot easily test a kpc2000, but
	perhaps `git blame` and talking to the authors would help
	figure out a way to validate the changes.

	Thinking about whether you can run a test that would prove or
	disprove my claim in (a), above, could be useful in coming up
	with tests to run.

In other words, a few very high quality conversions (even just one) that
we can really put our faith in, is what I value most here. Tested patches
are awesome.

3. Once I re-post, turn on the new CONFIG_DEBUG_GET_USER_PAGES_REFERENCES
and run things such as xfstest/fstest. (Again, doing so would be going
further than I have yet--very helpful). Help clarify what conversions have
actually been tested and work, and which ones remain unvalidated.

Other: Please note that this:

    https://github.com/johnhubbard/linux/commits/gup_dma_core

    a) gets rebased often, and

    b) has a bunch of commits (iov_iter and related) that conflict
       with the latest linux.git,

    c) has some bugs in the bio area, that I'm fixing, so I don't trust
       that's it's safely runnable, for a few more days.

One note below, for the future:

> 
> This is a bunch of trivial conversions which is a part of an effort
> by John Hubbard to solve issues with gup pinned pages and 
> filesystem writeback.
> 
> The issue is more clearly described in John Hubbard's patch[1] where
> put_user_page*() functions are introduced.
> 
> Currently put_user_page*() simply does put_page but future implementations
> look to change that once treewide change of put_page callsites to 
> put_user_page*() is finished.
> 
> The lwn article describing the issue with gup pinned pages and filesystem 
> writeback [2].
> 
> This patch has been tested by building and booting the kernel as I don't
> have the required hardware to test the device drivers.
> 
> I did not modify gpu/drm drivers which use release_pages instead of
> put_page() to release reference of gup pinned pages as I am not clear
> whether release_pages and put_page are interchangable. 
> 
> [1] https://lkml.org/lkml/2019/3/26/1396

When referring to patches in a commit description, please use the 
commit hash, not an external link. See Submitting Patches [1] for details.

Also, once you figure out the right maintainers and other involved people,
putting Cc: in the commit description is common practice, too.

[1] https://www.kernel.org/doc/html/latest/process/submitting-patches.html

thanks,
-- 
John Hubbard
NVIDIA

> 
> [2] https://lwn.net/Articles/784574/
> 
> Signed-off-by: Bharath Vedartham <linux.bhar@xxxxxxxxx>
> ---
>  drivers/media/v4l2-core/videobuf-dma-sg.c | 3 +--
>  drivers/misc/sgi-gru/grufault.c           | 2 +-
>  drivers/staging/kpc2000/kpc_dma/fileops.c | 4 +---
>  drivers/vfio/vfio_iommu_type1.c           | 2 +-
>  fs/io_uring.c                             | 7 +++----
>  mm/gup_benchmark.c                        | 6 +-----
>  net/xdp/xdp_umem.c                        | 7 +------
>  7 files changed, 9 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
> index 66a6c6c..d6eeb43 100644
> --- a/drivers/media/v4l2-core/videobuf-dma-sg.c
> +++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
> @@ -349,8 +349,7 @@ int videobuf_dma_free(struct videobuf_dmabuf *dma)
>  	BUG_ON(dma->sglen);
>  
>  	if (dma->pages) {
> -		for (i = 0; i < dma->nr_pages; i++)
> -			put_page(dma->pages[i]);
> +		put_user_pages(dma->pages, dma->nr_pages);
>  		kfree(dma->pages);
>  		dma->pages = NULL;
>  	}
> diff --git a/drivers/misc/sgi-gru/grufault.c b/drivers/misc/sgi-gru/grufault.c
> index 4b713a8..61b3447 100644
> --- a/drivers/misc/sgi-gru/grufault.c
> +++ b/drivers/misc/sgi-gru/grufault.c
> @@ -188,7 +188,7 @@ static int non_atomic_pte_lookup(struct vm_area_struct *vma,
>  	if (get_user_pages(vaddr, 1, write ? FOLL_WRITE : 0, &page, NULL) <= 0)
>  		return -EFAULT;
>  	*paddr = page_to_phys(page);
> -	put_page(page);
> +	put_user_page(page);
>  	return 0;
>  }
>  
> diff --git a/drivers/staging/kpc2000/kpc_dma/fileops.c b/drivers/staging/kpc2000/kpc_dma/fileops.c
> index 6166587..26dceed 100644
> --- a/drivers/staging/kpc2000/kpc_dma/fileops.c
> +++ b/drivers/staging/kpc2000/kpc_dma/fileops.c
> @@ -198,9 +198,7 @@ int  kpc_dma_transfer(struct dev_private_data *priv, struct kiocb *kcb, unsigned
>  	sg_free_table(&acd->sgt);
>   err_dma_map_sg:
>   err_alloc_sg_table:
> -	for (i = 0 ; i < acd->page_count ; i++){
> -		put_page(acd->user_pages[i]);
> -	}
> +	put_user_pages(acd->user_pages, acd->page_count);
>   err_get_user_pages:
>  	kfree(acd->user_pages);
>   err_alloc_userpages:
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index add34ad..c491524 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -369,7 +369,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>  		 */
>  		if (ret > 0 && vma_is_fsdax(vmas[0])) {
>  			ret = -EOPNOTSUPP;
> -			put_page(page[0]);
> +			put_user_page(page[0]);
>  		}
>  	}
>  	up_read(&mm->mmap_sem);
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 4ef62a4..b4a4549 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2694,10 +2694,9 @@ static int io_sqe_buffer_register(struct io_ring_ctx *ctx, void __user *arg,
>  			 * if we did partial map, or found file backed vmas,
>  			 * release any pages we did get
>  			 */
> -			if (pret > 0) {
> -				for (j = 0; j < pret; j++)
> -					put_page(pages[j]);
> -			}
> +			if (pret > 0)
> +				put_user_pages(pages, pret);
> +
>  			if (ctx->account_mem)
>  				io_unaccount_mem(ctx->user, nr_pages);
>  			kvfree(imu->bvec);
> diff --git a/mm/gup_benchmark.c b/mm/gup_benchmark.c
> index 7dd602d..15fc7a2 100644
> --- a/mm/gup_benchmark.c
> +++ b/mm/gup_benchmark.c
> @@ -76,11 +76,7 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
>  	gup->size = addr - gup->addr;
>  
>  	start_time = ktime_get();
> -	for (i = 0; i < nr_pages; i++) {
> -		if (!pages[i])
> -			break;
> -		put_page(pages[i]);
> -	}
> +	put_user_pages(pages, nr_pages);
>  	end_time = ktime_get();
>  	gup->put_delta_usec = ktime_us_delta(end_time, start_time);
>  
> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> index 9c6de4f..6103e19 100644
> --- a/net/xdp/xdp_umem.c
> +++ b/net/xdp/xdp_umem.c
> @@ -173,12 +173,7 @@ static void xdp_umem_unpin_pages(struct xdp_umem *umem)
>  {
>  	unsigned int i;
>  
> -	for (i = 0; i < umem->npgs; i++) {
> -		struct page *page = umem->pgs[i];
> -
> -		set_page_dirty_lock(page);
> -		put_page(page);
> -	}
> +	put_user_pages_dirty_lock(umem->pgs, umem->npgs);
>  
>  	kfree(umem->pgs);
>  	umem->pgs = NULL;
>