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Subject: Re: [PATCH RFC 2/4] mm/page_alloc: place pages to tail in
 __putback_isolated_page()
To:     Wei Yang <richard.weiyang@linux.alibaba.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-hyperv@vger.kernel.org, xen-devel@lists.xenproject.org,
        linux-acpi@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@suse.de>,
        Mike Rapoport <rppt@kernel.org>,
        Scott Cheloha <cheloha@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>
References: <20200916183411.64756-1-david@redhat.com>
 <20200916183411.64756-3-david@redhat.com>
 <20200918021654.GC54754@L-31X9LVDL-1304.local>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <a9d38779-73c5-73e9-aa7d-e26b87f6dbbb@redhat.com>
Date:   Fri, 18 Sep 2020 09:29:06 +0200
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On 18.09.20 04:16, Wei Yang wrote:
> On Wed, Sep 16, 2020 at 08:34:09PM +0200, David Hildenbrand wrote:
>> __putback_isolated_page() already documents that pages will be placed to
>> the tail of the freelist - this is, however, not the case for
>> "order >= MAX_ORDER - 2" (see buddy_merge_likely()) - which should be
>> the case for all existing users.
>>
>> This change affects two users:
>> - free page reporting
>> - page isolation, when undoing the isolation.
>>
>> This behavior is desireable for pages that haven't really been touched
>> lately, so exactly the two users that don't actually read/write page
>> content, but rather move untouched pages.
>>
>> The new behavior is especially desirable for memory onlining, where we
>> allow allocation of newly onlined pages via undo_isolate_page_range()
>> in online_pages(). Right now, we always place them to the head of the
>> free list, resulting in undesireable behavior: Assume we add
>> individual memory chunks via add_memory() and online them right away to
>> the NORMAL zone. We create a dependency chain of unmovable allocations
>> e.g., via the memmap. The memmap of the next chunk will be placed onto
>> previous chunks - if the last block cannot get offlined+removed, all
>> dependent ones cannot get offlined+removed. While this can already be
>> observed with individual DIMMs, it's more of an issue for virtio-mem
>> (and I suspect also ppc DLPAR).
>>
>> Note: If we observe a degradation due to the changed page isolation
>> behavior (which I doubt), we can always make this configurable by the
>> instance triggering undo of isolation (e.g., alloc_contig_range(),
>> memory onlining, memory offlining).
>>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Dave Hansen <dave.hansen@intel.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Mike Rapoport <rppt@kernel.org>
>> Cc: Scott Cheloha <cheloha@linux.ibm.com>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/page_alloc.c | 10 +++++++++-
>> 1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 91cefb8157dd..bba9a0f60c70 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -89,6 +89,12 @@ typedef int __bitwise fop_t;
>>  */
>> #define FOP_SKIP_REPORT_NOTIFY	((__force fop_t)BIT(0))
>>
>> +/*
>> + * Place the freed page to the tail of the freelist after buddy merging. Will
>> + * get ignored with page shuffling enabled.
>> + */
>> +#define FOP_TO_TAIL		((__force fop_t)BIT(1))
>> +
>> /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
>> static DEFINE_MUTEX(pcp_batch_high_lock);
>> #define MIN_PERCPU_PAGELIST_FRACTION	(8)
>> @@ -1040,6 +1046,8 @@ static inline void __free_one_page(struct page *page, unsigned long pfn,
>>
>> 	if (is_shuffle_order(order))
>> 		to_tail = shuffle_pick_tail();
>> +	else if (fop_flags & FOP_TO_TAIL)
>> +		to_tail = true;
> 
> Take another look into this part. Maybe we can move this check at top?
> 
> For online_page case, currently we have following call flow:
> 
>     online_page
>         online_pages_range
> 	shuffle_zone
> 
> This means we would always shuffle the newly added pages. Maybe we don't need
> to do the shuffle when adding them to the free_list?

Yeah we don't, but it doesn't really buy us too much as the call paths I
am touching are used by other mechanisms as well that need it
(especially undoing page isolation).

-- 
Thanks,

David / dhildenb