linux-kernel - Re: [PATCH 1/9] mm: Allow the [page|pfn]_mkwrite callbacks to drop the mmap

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Date:   Wed, 17 Apr 2019 10:58:42 +0000
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     "jrdr.linux@...il.com" <jrdr.linux@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "will.deacon@....com" <will.deacon@....com>,
        Linux-graphics-maintainer <Linux-graphics-maintainer@...are.com>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "ying.huang@...el.com" <ying.huang@...el.com>,
        "riel@...riel.com" <riel@...riel.com>
Subject: Re: [PATCH 1/9] mm: Allow the [page|pfn]_mkwrite callbacks to drop
 the mmap_sem

Hi, Souptick,

On Sat, 2019-04-13 at 20:41 +0530, Souptick Joarder wrote:
> On Fri, Apr 12, 2019 at 9:34 PM Thomas Hellstrom <
> thellstrom@...are.com> wrote:
> > Driver fault callbacks are allowed to drop the mmap_sem when
> > expecting
> > long hardware waits to avoid blocking other mm users. Allow the
> > mkwrite
> > callbacks to do the same by returning early on VM_FAULT_RETRY.
> > 
> > In particular we want to be able to drop the mmap_sem when waiting
> > for
> > a reservation object lock on a GPU buffer object. These locks may
> > be
> > held while waiting for the GPU.
> > 
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Matthew Wilcox <willy@...radead.org>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Rik van Riel <riel@...riel.com>
> > Cc: Minchan Kim <minchan@...nel.org>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Cc: Huang Ying <ying.huang@...el.com>
> > Cc: Souptick Joarder <jrdr.linux@...il.com>
> > Cc: "Jérôme Glisse" <jglisse@...hat.com>
> > Cc: linux-mm@...ck.org
> > Cc: linux-kernel@...r.kernel.org
> > 
> > Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
> > ---
> >  mm/memory.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index e11ca9dd823f..a95b4a3b1ae2 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2144,7 +2144,7 @@ static vm_fault_t do_page_mkwrite(struct
> > vm_fault *vmf)
> >         ret = vmf->vma->vm_ops->page_mkwrite(vmf);
> >         /* Restore original flags so that caller is not surprised
> > */
> >         vmf->flags = old_flags;
> > -       if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
> > +       if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_RETRY |
> > VM_FAULT_NOPAGE)))
> 
> With this patch there will multiple instances of (VM_FAULT_ERROR |
> VM_FAULT_RETRY | VM_FAULT_NOPAGE)
> in mm/memory.c. Does it make sense to wrap it in a macro and use it ?

Even though the code will look neater, it might be trickier to follow a
particular error path. Could we perhaps postpone to a follow-up patch?

Thomas



> 
> >                 return ret;
> >         if (unlikely(!(ret & VM_FAULT_LOCKED))) {
> >                 lock_page(page);
> > @@ -2419,7 +2419,7 @@ static vm_fault_t wp_pfn_shared(struct
> > vm_fault *vmf)
> >                 pte_unmap_unlock(vmf->pte, vmf->ptl);
> >                 vmf->flags |= FAULT_FLAG_MKWRITE;
> >                 ret = vma->vm_ops->pfn_mkwrite(vmf);
> > -               if (ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))
> > +               if (ret & (VM_FAULT_ERROR | VM_FAULT_RETRY |
> > VM_FAULT_NOPAGE))
> >                         return ret;
> >                 return finish_mkwrite_fault(vmf);
> >         }
> > @@ -2440,7 +2440,8 @@ static vm_fault_t wp_page_shared(struct
> > vm_fault *vmf)
> >                 pte_unmap_unlock(vmf->pte, vmf->ptl);
> >                 tmp = do_page_mkwrite(vmf);
> >                 if (unlikely(!tmp || (tmp &
> > -                                     (VM_FAULT_ERROR |
> > VM_FAULT_NOPAGE)))) {
> > +                                     (VM_FAULT_ERROR |
> > VM_FAULT_RETRY |
> > +                                      VM_FAULT_NOPAGE)))) {
> >                         put_page(vmf->page);
> >                         return tmp;
> >                 }
> > @@ -3494,7 +3495,8 @@ static vm_fault_t do_shared_fault(struct
> > vm_fault *vmf)
> >                 unlock_page(vmf->page);
> >                 tmp = do_page_mkwrite(vmf);
> >                 if (unlikely(!tmp ||
> > -                               (tmp & (VM_FAULT_ERROR |
> > VM_FAULT_NOPAGE)))) {
> > +                               (tmp & (VM_FAULT_ERROR |
> > VM_FAULT_RETRY |
> > +                                       VM_FAULT_NOPAGE)))) {
> >                         put_page(vmf->page);
> >                         return tmp;
> >                 }
> > --
> > 2.20.1
> >