linux-kernel - Re: mlock() on DAX returns -ENOMEM

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CACTTzNZJzsisnPVb_+6e2QHeoDC_q=pwD5eqe5NxDTLrFBW32w@mail.gmail.com>
Date:	Tue, 14 Apr 2015 13:27:39 +0300
From:	Yigal Korman <yigal@...xistor.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Matthew Wilcox <willy@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: mlock() on DAX returns -ENOMEM

On Mon, Apr 13, 2015 at 3:56 PM, Kirill A. Shutemov
<kirill@...temov.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.

>
> 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?

>
>> 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
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
>> the body of a message to majordomo@...r.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
> --
>  Kirill A. Shutemov

Thanks,
Yigal
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/