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

[Jerome Glisse <jglisse@redhat.com>]

On Fri, Jan 18, 2019 at 11:16:08AM +1100, Dave Chinner wrote:
> On Thu, Jan 17, 2019 at 10:21:08AM -0500, Jerome Glisse wrote:
> > On Wed, Jan 16, 2019 at 09:42:25PM -0800, John Hubbard wrote:
> > > On 1/16/19 5:08 AM, Jerome Glisse wrote:
> > > > On Wed, Jan 16, 2019 at 12:38:19PM +0100, Jan Kara wrote:
> > > >> That actually touches on another question I wanted to get opinions on. GUP
> > > >> can be for read and GUP can be for write (that is one of GUP flags).
> > > >> Filesystems with page cache generally have issues only with GUP for write
> > > >> as it can currently corrupt data, unexpectedly dirty page etc.. DAX & memory
> > > >> hotplug have issues with both (DAX cannot truncate page pinned in any way,
> > > >> memory hotplug will just loop in kernel until the page gets unpinned). So
> > > >> we probably want to track both types of GUP pins and page-cache based
> > > >> filesystems will take the hit even if they don't have to for read-pins?
> > > > 
> > > > Yes the distinction between read and write would be nice. With the map
> > > > count solution you can only increment the mapcount for GUP(write=true).
> > > > With pin bias the issue is that a big number of read pin can trigger
> > > > false positive ie you would do:
> > > >     GUP(vaddr, write)
> > > >         ...
> > > >         if (write)
> > > >             atomic_add(page->refcount, PAGE_PIN_BIAS)
> > > >         else
> > > >             atomic_inc(page->refcount)
> > > > 
> > > >     PUP(page, write)
> > > >         if (write)
> > > >             atomic_add(page->refcount, -PAGE_PIN_BIAS)
> > > >         else
> > > >             atomic_dec(page->refcount)
> > > > 
> > > > I am guessing false positive because of too many read GUP is ok as
> > > > it should be unlikely and when it happens then we take the hit.
> > > > 
> > > 
> > > I'm also intrigued by the point that read-only GUP is harmless, and we 
> > > could just focus on the writeable case.
> > 
> > For filesystem anybody that just look at the page is fine, as it would
> > not change its content thus the page would stay stable.
> 
> Other processes can access and dirty the page cache page while there
> is a GUP reference.  It's unclear to me whether that changes what
> GUP needs to do here, but we can't assume a page referenced for
> read-only GUP will be clean and unchanging for the duration of the
> GUP reference. It may even be dirty at the time of the read-only
> GUP pin...
> 

Yes and it is fine, GUP read only user do not assume that the page
is read only for everyone, it just means that the GUP user swear
it will only read from the page, not write to it.

So for GUP read only we do not need to synchronize with anything
writting to the page.

Cheers,
Jérôme