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? -- 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
