[Add Pavel - the email thread starts http://lkml.kernel.org/r/20191030131122.8256-1-vincent.whitchurch@xxxxxxxx but it used your old email address] On Wed 30-10-19 15:02:16, Vincent Whitchurch wrote: > On Wed, Oct 30, 2019 at 02:29:58PM +0100, Michal Hocko wrote: > > On Wed 30-10-19 14:11:22, Vincent Whitchurch wrote: > > > (I noticed this because on my ARM64 platform, with 1 GiB of memory the > > > first [and only] section is allocated from the zeroing path while with > > > 2 GiB of memory the first 1 GiB section is allocated from the > > > non-zeroing path.) > > > > Do I get it right that sparse_buffer_init couldn't allocate memmap for > > the full node for some reason and so sparse_init_nid would have to > > allocate one for each memory section? > > Not quite. The sparsemap_buf is successfully allocated with the correct > size in sparse_buffer_init(), but sparse_buffer_alloc() fails to > allocate the same size from it. > > The reason it fails is that sparse_buffer_alloc() for some reason wants > to return a pointer which is aligned to the allocation size. But the > sparsemap_buf was only allocated with PAGE_SIZE alignment so there's not > enough space to align it. > > I don't understand the reason for this alignment requirement since the > fallback path also allocates with PAGE_SIZE alignment. I'm guessing the > alignment is for the VMEMAP code which also uses sparse_buffer_alloc()? I am not 100% sure TBH. Aligning makes some sense when mapping the memmaps to page tables but that would suggest that sparse_buffer_init is using a wrong alignment then. It is quite wasteful to allocate alarge misaligned block like that. Your patch still makes sense but this is something to look into. Pavel? -- Michal Hocko SUSE Labs
