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@xxxxxxxxxxxxx> 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 > >> -- > >> To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > >> the body of a message to majordomo@xxxxxxxxxxxxxxx > >> 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@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
