linux-kernel - Re: [PATCH 0/2] mm/fs: put_user

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Date:   Mon, 9 Jul 2018 18:27:58 +0200
From:   Jan Kara <jack@...e.cz>
To:     john.hubbard@...il.com
Cc:     Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 0/2] mm/fs: put_user_page() proposal

Hi,

On Mon 09-07-18 01:05:52, john.hubbard@...il.com wrote:
> From: John Hubbard <jhubbard@...dia.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@...e.com>
SUSE Labs, CR