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

[Dan Williams]

On Wed, Dec 12, 2018 at 7:03 AM Jerome Glisse <jglisse@xxxxxxxxxx> 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:
> > > Another crazy idea, why not treating GUP as another mapping of the page
> > > and caller of GUP would have to provide either a fake anon_vma struct or
> > > a fake vma struct (or both for PRIVATE mapping of a file where you can
> > > have a mix of both private and file page thus only if it is a read only
> > > GUP) that would get added to the list of existing mapping.
> > >
> > > So the flow would be:
> > >     somefunction_thatuse_gup()
> > >     {
> > >         ...
> > >         GUP(_fast)(vma, ..., fake_anon, fake_vma);
> > >         ...
> > >     }
> > >
> > >     GUP(vma, ..., fake_anon, fake_vma)
> > >     {
> > >         if (vma->flags == ANON) {
> > >             // Add the fake anon vma to the anon vma chain as a child
> > >             // of current vma
> > >         } else {
> > >             // Add the fake vma to the mapping tree
> > >         }
> > >
> > >         // The existing GUP except that now it inc mapcount and not
> > >         // refcount
> > >         GUP_old(..., &nanonymous, &nfiles);
> > >
> > >         atomic_add(&fake_anon->refcount, nanonymous);
> > >         atomic_add(&fake_vma->refcount, nfiles);
> > >
> > >         return nanonymous + nfiles;
> > >     }
> >
> > Thanks for your idea! This is actually something like I was suggesting back
> > at LSF/MM in Deer Valley. There were two downsides to this I remember
> > people pointing out:
> >
> > 1) This cannot really work with __get_user_pages_fast(). You're not allowed
> > to get necessary locks to insert new entry into the VMA tree in that
> > context. So essentially we'd loose get_user_pages_fast() functionality.
> >
> > 2) The overhead e.g. for direct IO may be noticeable. You need to allocate
> > the fake tracking VMA, get VMA interval tree lock, insert into the tree.
> > Then on IO completion you need to queue work to unpin the pages again as you
> > cannot remove the fake VMA directly from interrupt context where the IO is
> > completed.
> >
> > You are right that the cost could be amortized if gup() is called for
> > multiple consecutive pages however for small IOs there's no help...
> >
> > 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).
>
>     [O3] DAX and the device block problems, ie with DAX the page map in
>          userspace is the same as the block (persistent memory) and no
>          filesystem nor block device understand page as block or pinned
>          block.
>
> For [O3] i don't think any pin count would help in anyway. I believe
> that the current long term GUP API that does not allow GUP of DAX is
> the only sane solution for now.

No, that's not a sane solution, it's an emergency hack.

> The real fix would be to teach file-
> system about DAX/pinned block so that a pinned block is not reuse
> by filesystem.

We already have taught filesystems about pinned dax pages, see
dax_layout_busy_page(). As much as possible I want to eliminate the
concept of "dax pages" as a special case that gets sprinkled
throughout the mm.

> 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

With get_user_pages_fast() we don't know that we have a file-backed
page, because we don't have a vma.