On Mon, Jun 22, 2020 at 04:06:15PM +0200, David Hildenbrand wrote:
On 22.06.20 15:10, Wei Yang wrote:
On Mon, Jun 22, 2020 at 11:51:34AM +0200, David Hildenbrand wrote:
On 22.06.20 11:22, Wei Yang wrote:
On Mon, Jun 22, 2020 at 10:43:11AM +0200, David Hildenbrand wrote:
On 22.06.20 10:26, Wei Yang wrote:
On Fri, Jun 19, 2020 at 02:59:20PM +0200, David Hildenbrand wrote: > Especially with memory hotplug, we can have offline sections (with a > garbage memmap) and overlapping zones. We have to make sure to only > touch initialized memmaps (online sections managed by the buddy) and that > the zone matches, to not move pages between zones. > > To test if this can actually happen, I added a simple > BUG_ON(page_zone(page_i) != page_zone(page_j)); > right before the swap. When hotplugging a 256M DIMM to a 4G x86-64 VM and > onlining the first memory block "online_movable" and the second memory > block "online_kernel", it will trigger the BUG, as both zones (NORMAL > and MOVABLE) overlap. > > This might result in all kinds of weird situations (e.g., double > allocations, list corruptions, unmovable allocations ending up in the > movable zone). > > Fixes: e900a918b098 ("mm: shuffle initial free memory to improve memory-side-cache utilization") > Acked-by: Michal Hocko mhocko@suse.com > Cc: stable@vger.kernel.org # v5.2+ > Cc: Andrew Morton akpm@linux-foundation.org > Cc: Johannes Weiner hannes@cmpxchg.org > Cc: Michal Hocko mhocko@suse.com > Cc: Minchan Kim minchan@kernel.org > Cc: Huang Ying ying.huang@intel.com > Cc: Wei Yang richard.weiyang@gmail.com > Cc: Mel Gorman mgorman@techsingularity.net > Signed-off-by: David Hildenbrand david@redhat.com > --- > mm/shuffle.c | 18 +++++++++--------- > 1 file changed, 9 insertions(+), 9 deletions(-) > > diff --git a/mm/shuffle.c b/mm/shuffle.c > index 44406d9977c77..dd13ab851b3ee 100644 > --- a/mm/shuffle.c > +++ b/mm/shuffle.c > @@ -58,25 +58,25 @@ module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400); > * For two pages to be swapped in the shuffle, they must be free (on a > * 'free_area' lru), have the same order, and have the same migratetype. > */ > -static struct page * __meminit shuffle_valid_page(unsigned long pfn, int order) > +static struct page * __meminit shuffle_valid_page(struct zone *zone, > + unsigned long pfn, int order) > { > - struct page *page; > + struct page *page = pfn_to_online_page(pfn);
Hi, David and Dan,
One thing I want to confirm here is we won't have partially online section, right? We can add a sub-section to system, but we won't manage it by buddy.
Hi,
there is still a BUG with sub-section hot-add (devmem), which broke pfn_to_online_page() in corner cases (especially, see the description in include/linux/mmzone.h). We can have a boot-memory section partially populated and marked online. Then, we can hot-add devmem, marking the remaining pfns valid - and as the section is maked online, also as online.
Oh, yes, I see this description.
This means we could have section marked as online, but with a sub-section even not added.
While the good news is even the sub-section is not added, but its memmap is populated for an early section. So the page returned from pfn_to_online_page() is a valid one.
But what would happen, if the sub-section is removed after added? Would section_deactivate() release related memmap to this "struct page"?
If devmem is removed, the memmap will be freed and the sub-sections are marked as non-present. So this works as expected.
Sorry, I may not catch your point. If my understanding is correct, the above behavior happens in function section_deactivate().
Let me draw my understanding of function section_deactivate():
section_deactivate(pfn, nr_pages) clear_subsection_map(pfn, nr_pages)depopulate_section_memmap(pfn, nr_pages)
Since we just remove a sub-section, I skipped some un-related codes. These two functions would:
- clear bitmap in ms->usage->subsection_map
- free memmap for the sub-section
While since the section is not empty, ms->section_mem_map is not set no null.
Let me clarify, sub-section hotremove works differently when overlying with (online) boot memory within a section.
Early sections (IOW, boot memory) are never partially removed. See
Thanks for your time and patience.
Looked into the comment of section_deactivate():
* 1. deactivation of a partial hot-added section (only possible in * the SPARSEMEM_VMEMMAP=y case). * a) section was present at memory init. * b) section was hot-added post memory init.
Case a) seems do partial remove for an early section?
mm/sparse.c:section_deactivate(). We only free a early memmap when the section is completely empty. Also see how
Hmm.. I thought this is the behavior for early section, while it looks current code doesn't work like this:
if (section_is_early && memmap) free_map_bootmem(memmap); else depopulate_section_memmap(pfn, nr_pages, altmap);
section_is_early is always "true" for early section, while memmap is not-NULL only when sub-section map is empty.
If my understanding is correct, when we remove a sub-section in early section, the code would call depopulate_section_memmap(), which in turn free related memmap. By removing the memmap, the return value from pfn_to_online_page() is not a valid one.
Maybe we want to write the code like this:
if (section_is_early) if (memmap) free_map_bootmem(memmap); else depopulate_section_memmap(pfn, nr_pages, altmap);
This makes sure we only free memmap for early section only when the whole section is removed.
include/linux/mmzone.h:pfn_valid() handles early sections.
So when we have a partially present section with boot memory, we a) marked the whole section present and online (there is only a single bit) b) allocated the memmap for the whole section c) Only exposed the relevant pages to the buddy. The memmap of non- present parts in a section were initialized and are reserved.
pfn_valid() will return for all non-present pfns valid, because there is a memmap. pfn_to_online_page() will return for all pfns "true", because we only have a single bit for the whole section. This has been the case before sub-section hotplug and is still the case. It simply looks like just another memory hole for which we have a memmap.
Now, with devmem it is possible to suddenly change these sub-section holes (memmaps) to become ZONE_DEVICE memory. pfn_to_online_page() would have to detect that and report a "false". Possible fixes were already discussed (e.g., sub-section online map instead of a single bit).
Again, the zone check safes us from the worst, just as in the case of all other pfn walkers that use (as documented) pfn_to_online_page(). It still needs a fix as dicussed, but it seems to work reasonably fine like that for now.
-- Thanks,
David / dhildenb