Re: mlock() on DAX returns -ENOMEM

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Apr 14, 2015 at 01:27:39PM +0300, Yigal Korman wrote:
> On Mon, Apr 13, 2015 at 3:56 PM, Kirill A. Shutemov
> <kirill@shutemov.name> wrote:
> > On Sun, Apr 12, 2015 at 03:56:33PM +0300, Yigal Korman wrote:
> >> Hi,
> >> I've tried to mlock() a range of an ext4-dax file and got "-ENOMEM" in return.
> >
> > Is it comes from mlock_fixup() or -EFAULT from GUP translated to -ENOMEM
> > by __mlock_posix_error_return()?
> 
> It comes from GUP. An associate followed the flow from GUP to
> vm_normal_page which returns NULL for VM_MIXEDMAP.
> 
> >
> >> Looking at the code, it seems that this is related to the fact that
> >> DAX uses VM_MIXEDMAP and mlock assumes/requires regular page cache.
> >> To me it seems that DAX should simply return success in mlock() as all
> >> data is always in memory and no swapping is possible.
> >> Is this a bug or intentional? Is there a fix planned?
> >
> > I think it's a bug.
> >
> > But first we need to define what mlock() means for DAX mappings.
> >
> > For writable MAP_PRIVATE: we should be able to trigger COW for the range
> > and mlock resulting pages. It means we should fix kernel to handle
> > GUP(FOLL_TOUCH | FOLL_POPULATE | FOLL_WRITE | FOLL_FORCE) successfully on
> > such VMAs.
> 
> writable MAP_PRIVATE seems to work OK already.

Really? Have you try mlock() on writable MAP_PRIVATE mappings after some
*read* read faults to the VMA?

I suspect mlock() will fail if VMA was populated with PFNs before mlock()
call.

> 
> >
> > For MAP_SHARED and non-writable MAP_PRIVATE we should be able to populate
> > the mapping with PTEs. Not sure if we need to set VM_LOCKED for such VMAs.
> > We probably should, as we want to re-instantiate PTEs on mremap() and such.
> > It means we need to get working at least GUP(FOLL_POPULATE | FOLL_FORCE).
> >
> > In general we need to adjust GUP to avoid going to struct page unless
> > FOLL_* speficly imply struct page, such as FOLL_GET or FOLL_TOUCH.
> >
> > Not sure if we need to differentiate DAX mappings from other VM_MIXEDMAP.
> >
> > Any comments?
> 
> Indeed, the issue is VM_MIXEDMAP and not DAX specifically.
> What will happen for other users of VM_MIXEDMAP that, unlike DAX, do
> swap pages and need some special callback to handle mlock?

Do far only DRM drivers uses VM_MIXEDMAP. It shouldn't be too hard to
audit this code.

Or we can identify DAX mapping some other way. Or introduce VM_DAX.
 
> >
> >> Also, the same code path that is used in mlock is also used for
> >> MAP_POPULATE (pre-fault pages in mmap) so this flag doesn't work as
> >> well (doesn't fail but simply doesn't pre-fault anything).
> >>
> >> Thanks,
> >> Yigal
> >> --
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> >
> > --
> >  Kirill A. Shutemov
> 
> Thanks,
> Yigal
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-- 
 Kirill A. Shutemov

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