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

[Octavian Purdila]

On Fri, Nov 20, 2015 at 5:11 PM, Brian Foster <bfoster@xxxxxxxxxx> 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@xxxxxxxxx>
>> > > ---
>> >
>> > 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?
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