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SubjectRe: [PATCH 0/2] mm/fs: put_user_page() proposal
Hi,

On Mon 09-07-18 01:05:52, john.hubbard@gmail.com wrote:
> From: John Hubbard <jhubbard@nvidia.com>
>
> With respect to tracking get_user_pages*() pages with page->dma_pinned*
> fields [1], I spent a few days retrofitting most of the get_user_pages*()
> call sites, by adding calls to a new put_user_page() function, in place
> of put_page(), where appropriate. This will work, but it's a large effort.
>
> Design note: I didn't see anything that hinted at a way to fix this
> problem, without actually changing all of the get_user_pages*() call sites,
> so I think it's reasonable to start with that.

Agreed.

> Anyway, it's still incomplete, but because this is a large, tree-wide
> change (that will take some time and testing), I'd like to propose a plan,
> before spamming zillions of people with put_user_page() conversion patches.
> So I picked out the first two patches to show where this is going.
>
> Proposed steps:
>
> Step 1:
>
> Start with the patches here, then continue with...dozens more.
> This will eventually convert all of the call sites to use put_user_page().
> This is easy in some places, but complex in others, such as:
>
> -- drivers/gpu/drm/amd
> -- bio
> -- fuse
> -- cifs
> -- anything from:
> git grep iov_iter_get_pages | cut -f1 -d ':' | sort | uniq
>
> The easy ones can be grouped into a single patchset, perhaps, and the
> complex ones probably each need a patchset, in order to get the in-depth
> review they'll need.

Agreed.

> Furthermore, some of these areas I hope to attract some help on, once
> this starts going.
>
> Step 2:
>
> In parallel, tidy up the core patchset that was discussed in [1], (version
> 2 has already been reviewed, so I know what to do), and get it perfected
> and reviewed. Don't apply it until step 1 is all done, though.
>
> Step 3:
>
> Activate refcounting of dma-pinned pages (essentially, patch #5, which is
> [1]), but don't use it yet. Place a few WARN_ON_ONCE calls to start
> mopping up any missed call sites.
>
> Step 4:
>
> After some soak time, actually connect it up (patch #6 of [1]) and start
> taking action based on the new page->dma_pinned* fields.
>
> [1] https://www.spinics.net/lists/linux-mm/msg156409.html
>
> or, the same thread on LKML if it's working for you:
>
> https://lkml.org/lkml/2018/7/4/368

Yeah, but as Nick pointed out we have some more work to do in step 4 to
avoid deadlocks. Still there's a lot of work to do on which the direction
to progress is clear :).

Honza

--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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