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Message-ID: <20180408065055.GA19345@localhost.localdomain> Date: Sun, 8 Apr 2018 14:50:55 +0800 From: Baoquan He <bhe@...hat.com> To: Dave Hansen <dave.hansen@...el.com> Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org, pagupta@...hat.com, linux-mm@...ck.org, kirill.shutemov@...ux.intel.com Subject: Re: [PATCH v3 2/4] mm/sparsemem: Defer the ms->section_mem_map clearing Hi Dave, Thanks a lot for your careful reviewing! On 04/06/18 at 07:23am, Dave Hansen wrote: > On 02/27/2018 07:26 PM, Baoquan He wrote: > > In sparse_init(), if CONFIG_SPARSEMEM_ALLOC_MEM_MAP_TOGETHER=y, system > > will allocate one continuous memory chunk for mem maps on one node and > > populate the relevant page tables to map memory section one by one. If > > fail to populate for a certain mem section, print warning and its > > ->section_mem_map will be cleared to cancel the marking of being present. > > Like this, the number of mem sections marked as present could become > > less during sparse_init() execution. > > > > Here just defer the ms->section_mem_map clearing if failed to populate > > its page tables until the last for_each_present_section_nr() loop. This > > is in preparation for later optimizing the mem map allocation. > > > > Signed-off-by: Baoquan He <bhe@...hat.com> > > --- > > mm/sparse-vmemmap.c | 1 - > > mm/sparse.c | 12 ++++++++---- > > 2 files changed, 8 insertions(+), 5 deletions(-) > > > > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c > > index bd0276d5f66b..640e68f8324b 100644 > > --- a/mm/sparse-vmemmap.c > > +++ b/mm/sparse-vmemmap.c > > @@ -303,7 +303,6 @@ void __init sparse_mem_maps_populate_node(struct page **map_map, > > ms = __nr_to_section(pnum); > > pr_err("%s: sparsemem memory map backing failed some memory will not be available\n", > > __func__); > > - ms->section_mem_map = 0; > > } > > I think you might have been trying to say this in the description, but I > was not able to parse it out of there. What is in ms->section_mem_map > that needs to get cleared? > > It *looks* like memory_present() uses ms->section_mem_map to just mark > which sections are online relatively early in boot. We need this > clearing to mark that they are effectively *not* present any longer. > Correct? > > I guess the concern here is that if you miss any of the error sites, > we'll end up with a bogus, non-null ms->section_mem_map. Do we handle > that nicely? > > Should the " = 0" instead be clearing SECTION_MARKED_PRESENT or > something? That would make it easier to match the code up with the code > that it is effectively undoing. Not sure if I understand your question correctly. From memory_present(), information encoded into ms->section_mem_map including numa node, SECTION_IS_ONLINE and SECTION_MARKED_PRESENT. Not sure if it's OK to only clear SECTION_MARKED_PRESENT. People may wrongly check SECTION_IS_ONLINE and do something on this memory section? Thanks Baoquan
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