Re: mlock() on DAX returns -ENOMEM

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

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()?

> 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.

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?
 
> 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

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