Messages in this thread | | | Date | Mon, 2 Oct 2023 10:56:51 +0200 | Subject | Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY | From | David Hildenbrand <> |
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On 02.10.23 10:47, Mike Rapoport wrote: > On Mon, Oct 02, 2023 at 03:03:56PM +0800, Yajun Deng wrote: >> >> On 2023/10/2 02:59, Mike Rapoport wrote: >>> On Fri, Sep 29, 2023 at 06:27:25PM +0800, Yajun Deng wrote: >>>> On 2023/9/29 18:02, Mike Rapoport wrote: >>>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>>>>>> index 06be8821d833..b868caabe8dc 100644 >>>>>>>> --- a/mm/page_alloc.c >>>>>>>> +++ b/mm/page_alloc.c >>>>>>>> @@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order) >>>>>>>> unsigned int loop; >>>>>>>> /* >>>>>>>> - * When initializing the memmap, __init_single_page() sets the refcount >>>>>>>> - * of all pages to 1 ("allocated"/"not free"). We have to set the >>>>>>>> - * refcount of all involved pages to 0. >>>>>>>> + * When initializing the memmap, memmap_init_range sets the refcount >>>>>>>> + * of all pages to 1 ("reserved" and "free") in hotplug context. We >>>>>>>> + * have to set the refcount of all involved pages to 0. Otherwise, >>>>>>>> + * we don't do it, as reserve_bootmem_region only set the refcount on >>>>>>>> + * reserve region ("reserved") in early context. >>>>>>>> */ >>>>>>> Again, why hotplug and early init should be different? >>>>>> I will add a comment that describes it will save boot time. >>>>> But why do we need initialize struct pages differently at boot time vs >>>>> memory hotplug? >>>>> Is there a reason memory hotplug cannot have page count set to 0 just like >>>>> for pages reserved at boot time? >>>> This patch just save boot time in MEMINIT_EARLY. If someone finds out that >>>> it can save time in >>>> >>>> MEMINIT_HOTPLUG, I think it can be done in another patch later. I just >>>> keeping it in the same. >>> But it's not the same. It becomes slower after your patch and the code that >>> frees the pages for MEMINIT_EARLY and MEMINIT_HOTPLUG becomes non-uniform >>> for no apparent reason. >> >> __free_pages_core will also be called by others, such as: >> deferred_free_range, do_collection and memblock_free_late. >> >> We couldn't remove 'if (page_count(page))' even if we set page count to 0 >> when MEMINIT_HOTPLUG. > > That 'if' breaks the invariant that __free_pages_core is always called for > pages with initialized page count. Adding it may lead to subtle bugs and > random memory corruption so we don't want to add it at the first place.
As long as we have to special-case memory hotplug, we know that we are always coming via generic_online_page() in that case. We could either move some logic over there, or let __free_pages_core() know what it should do.
-- Cheers,
David / dhildenb
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