Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Dave Chinner <david@fromorbit.com>]

On Wed, Dec 12, 2018 at 10:03:20AM -0500, Jerome Glisse wrote:
> On Mon, Dec 10, 2018 at 11:28:46AM +0100, Jan Kara wrote:
> > On Fri 07-12-18 21:24:46, Jerome Glisse wrote:
> > So this approach doesn't look like a win to me over using counter in struct
> > page and I'd rather try looking into squeezing HMM public page usage of
> > struct page so that we can fit that gup counter there as well. I know that
> > it may be easier said than done...
> 
> So i want back to the drawing board and first i would like to ascertain
> that we all agree on what the objectives are:
> 
>     [O1] Avoid write back from a page still being written by either a
>          device or some direct I/O or any other existing user of GUP.
>          This would avoid possible file system corruption.
> 
>     [O2] Avoid crash when set_page_dirty() is call on a page that is
>          considered clean by core mm (buffer head have been remove and
>          with some file system this turns into an ugly mess).

I think that's wrong. This isn't an "avoid a crash" case, this is a
"prevent data and/or filesystem corruption" case. The primary goal
we have here is removing our exposure to potential corruption, which
has the secondary effect of avoiding the crash/panics that currently
occur as a result of inconsistent page/filesystem state.

i.e. The goal is to have ->page_mkwrite() called on the clean page
/before/ the file-backed page is marked dirty, and hence we don't
expose ourselves to potential corruption or crashes that are a
result of inappropriately calling set_page_dirty() on clean
file-backed pages.

> For [O1] and [O2] i believe a solution with mapcount would work. So
> no new struct, no fake vma, nothing like that. In GUP for file back
> pages we increment both refcount and mapcount (we also need a special
> put_user_page to decrement mapcount when GUP user are done with the
> page).

I don't see how a mapcount can prevent anyone from calling
set_page_dirty() inappropriately.

> Now for [O1] the write back have to call page_mkclean() to go through
> all reverse mapping of the page and map read only. This means that
> we can count the number of real mapping and see if the mapcount is
> bigger than that. If mapcount is bigger than page is pin and we need
> to use a bounce page to do the writeback.

Doesn't work. Generally filesystems have already mapped the page
into bios before they call clear_page_dirty_for_io(), so it's too
late for the filesystem to bounce the page at that point.

> For [O2] i believe we can handle that case in the put_user_page()
> function to properly dirty the page without causing filesystem
> freak out.

I'm pretty sure you can't call ->page_mkwrite() from
put_user_page(), so I don't think this is workable at all.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com