Re: [RFC PATCH] xfs: support for non-mmu architectures

[Brian Foster <bfoster@redhat.com>]

On Fri, Nov 20, 2015 at 05:35:55PM +0200, Octavian Purdila wrote:
> On Fri, Nov 20, 2015 at 5:11 PM, Brian Foster <bfoster@redhat.com> wrote:
> > On Fri, Nov 20, 2015 at 10:35:47AM +1100, Dave Chinner wrote:
> >> On Thu, Nov 19, 2015 at 10:55:25AM -0500, Brian Foster wrote:
> >> > On Wed, Nov 18, 2015 at 12:46:21AM +0200, Octavian Purdila wrote:
> >> > > Naive implementation for non-mmu architectures: allocate physically
> >> > > contiguous xfs buffers with alloc_pages. Terribly inefficient with
> >> > > memory and fragmentation on high I/O loads but it may be good enough
> >> > > for basic usage (which most non-mmu architectures will need).
> >> > >
> >> > > This patch was tested with lklfuse [1] and basic operations seems to
> >> > > work even with 16MB allocated for LKL.
> >> > >
> >> > > [1] https://github.com/lkl/linux
> >> > >
> >> > > Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
> >> > > ---
> >> >
> >> > Interesting, though this makes me wonder why we couldn't have a new
> >> > _XBF_VMEM (for example) buffer type that uses vmalloc(). I'm not
> >> > familiar with mmu-less context, but I see that mm/nommu.c has a
> >> > __vmalloc() interface that looks like it ultimately translates into an
> >> > alloc_pages() call. Would that accomplish what this patch is currently
> >> > trying to do?
> >>
> >> vmalloc is always a last resort.  vmalloc space on 32 bit systems is
> >> extremely limited and it is easy to exhaust with XFS.
> >>
> >
> > Sure, but my impression is that a vmalloc() buffer is roughly equivalent
> > in this regard to a current !XBF_UNMAPPED && size > PAGE_SIZE buffer. We
> > just do the allocation and mapping separately (presumably for other
> > reasons).
> >
> >> Also, vmalloc limits the control we have over allocation context
> >> (e.g. the hoops we jump through in kmem_alloc_large() to maintain
> >> GFP_NOFS contexts), so just using vmalloc doesn't make things much
> >> simpler from an XFS perspective.
> >>
> >
> > The comment in kmem_zalloc_large() calls out some apparent hardcoded
> > allocation flags down in the depths of vmalloc(). It looks to me that
> > page allocation (__vmalloc_area_node()) actually uses the provided
> > flags, so I'm not following the "data page" part of that comment.
> > Indeed, I do see that this is not the case down in calls like
> > pmd_alloc_one(), pte_alloc_one_kernel(), etc., associated with page
> > table management.
> >
> > Those latter calls are all from following down through the
> > map_vm_area()->vmap_page_range() codepath from __vmalloc_area_node(). We
> > call vm_map_ram() directly from _xfs_buf_map_pages(), which itself calls
> > down into the same code. Indeed, we already protect ourselves here via
> > the same memalloc_noio_save() mechanism that kmem_zalloc_large() uses.
> >
> > I suspect there's more to it than that because it does look like
> > vm_map_ram() has a different mechanism for managing vmalloc space for
> > certain (smaller) allocations, either of which I'm not really familiar
> > with. That aside, I don't see how vmalloc() introduces any new
> > allocation context issues for those buffers where we already set up a
> > multi-page mapping.
> >
> > We still have the somewhat customized page allocation code in
> > xfs_buf_allocate_memory() to contend with. I actually think it would be
> > useful to have a DEBUG sysfs tunable to turn on vmalloc() buffers and
> > actually test how effective some of this code is.
> >
> >> > I ask because it seems like that would help clean up the code a bit, for
> >> > one. It might also facilitate some degree of testing of the XFS bits
> >> > (even if utilized sparingly in DEBUG mode if it weren't suitable enough
> >> > for generic/mmu use). We currently allocate and map the buffer pages
> >> > separately and I'm not sure if there's any particular reasons for doing
> >> > that outside of some congestion handling in the allocation code and
> >> > XBF_UNMAPPED buffers, the latter probably being irrelevant for nommu.
> >> > Any other thoughts on that?
> >>
> >> We could probably clean the code up more (the allocation logic
> >> is now largely a historic relic) but I'm not convinced yet that we
> >> should be spending any time trying to specifically support mmu-less
> >> hardware.
> >>
> >
> > Fair point, we'll see where the use case discussion goes. That said, I
> > was a little surprised that this is all that was required to enable
> > nommu support. If that is indeed the case and we aren't in for a series
> > of subsequent nommu specific changes (Octavian?) by letting this
> > through, what's the big deal? This seems fairly harmless to me as is,
> > particularly if it can be semi-tested via DEBUG mode and has potential
> > generic use down the road.
> >
> 
> I don't foresee additional patches. I was able to use lklfuse to mount
> an XFS image and perform basic operations. Are there any xfs specific
> tests coverage tools I can use to make sure I am not missing anything?
> 

Ok, well you probably want to run some of the tests in xfstests and see
if anything falls over:

https://git.kernel.org/cgit/fs/xfs/xfstests-dev.git/

Note that this has generic as well as fs-specific tests for other
filesystems (ext4, btrfs, etc.).

Brian

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