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SubjectRe: [PATCH -mm -v4 03/21] mm, THP, swap: Support PMD swap mapping in swap_duplicate()
Dan Williams <> writes:

> On Thu, Jun 21, 2018 at 8:55 PM Huang, Ying <> wrote:
>> From: Huang Ying <>
>> To support to swapin the THP as a whole, we need to create PMD swap
>> mapping during swapout, and maintain PMD swap mapping count. This
>> patch implements the support to increase the PMD swap mapping
>> count (for swapout, fork, etc.) and set SWAP_HAS_CACHE flag (for
>> swapin, etc.) for a huge swap cluster in swap_duplicate() function
>> family. Although it only implements a part of the design of the swap
>> reference count with PMD swap mapping, the whole design is described
>> as follow to make it easy to understand the patch and the whole
>> picture.
>> A huge swap cluster is used to hold the contents of a swapouted THP.
>> After swapout, a PMD page mapping to the THP will become a PMD
>> swap mapping to the huge swap cluster via a swap entry in PMD. While
>> a PTE page mapping to a subpage of the THP will become the PTE swap
>> mapping to a swap slot in the huge swap cluster via a swap entry in
>> PTE.
>> If there is no PMD swap mapping and the corresponding THP is removed
>> from the page cache (reclaimed), the huge swap cluster will be split
>> and become a normal swap cluster.
>> The count (cluster_count()) of the huge swap cluster is
>> SWAPFILE_CLUSTER (= HPAGE_PMD_NR) + PMD swap mapping count. Because
>> all swap slots in the huge swap cluster are mapped by PTE or PMD, or
>> has SWAP_HAS_CACHE bit set, the usage count of the swap cluster is
>> HPAGE_PMD_NR. And the PMD swap mapping count is recorded too to make
>> it easy to determine whether there are remaining PMD swap mappings.
>> The count in swap_map[offset] is the sum of PTE and PMD swap mapping
>> count. This means when we increase the PMD swap mapping count, we
>> need to increase swap_map[offset] for all swap slots inside the swap
>> cluster. An alternative choice is to make swap_map[offset] to record
>> PTE swap map count only, given we have recorded PMD swap mapping count
>> in the count of the huge swap cluster. But this need to increase
>> swap_map[offset] when splitting the PMD swap mapping, that may fail
>> because of memory allocation for swap count continuation. That is
>> hard to dealt with. So we choose current solution.
>> The PMD swap mapping to a huge swap cluster may be split when unmap a
>> part of PMD mapping etc. That is easy because only the count of the
>> huge swap cluster need to be changed. When the last PMD swap mapping
>> is gone and SWAP_HAS_CACHE is unset, we will split the huge swap
>> cluster (clear the huge flag). This makes it easy to reason the
>> cluster state.
>> A huge swap cluster will be split when splitting the THP in swap
>> cache, or failing to allocate THP during swapin, etc. But when
>> splitting the huge swap cluster, we will not try to split all PMD swap
>> mappings, because we haven't enough information available for that
>> sometimes. Later, when the PMD swap mapping is duplicated or swapin,
>> etc, the PMD swap mapping will be split and fallback to the PTE
>> operation.
>> When a THP is added into swap cache, the SWAP_HAS_CACHE flag will be
>> set in the swap_map[offset] of all swap slots inside the huge swap
>> cluster backing the THP. This huge swap cluster will not be split
>> unless the THP is split even if its PMD swap mapping count dropped to
>> 0. Later, when the THP is removed from swap cache, the SWAP_HAS_CACHE
>> flag will be cleared in the swap_map[offset] of all swap slots inside
>> the huge swap cluster. And this huge swap cluster will be split if
>> its PMD swap mapping count is 0.
>> Signed-off-by: "Huang, Ying" <>
>> Cc: "Kirill A. Shutemov" <>
>> Cc: Andrea Arcangeli <>
>> Cc: Michal Hocko <>
>> Cc: Johannes Weiner <>
>> Cc: Shaohua Li <>
>> Cc: Hugh Dickins <>
>> Cc: Minchan Kim <>
>> Cc: Rik van Riel <>
>> Cc: Dave Hansen <>
>> Cc: Naoya Horiguchi <>
>> Cc: Zi Yan <>
>> Cc: Daniel Jordan <>
>> ---
>> include/linux/huge_mm.h | 5 +
>> include/linux/swap.h | 9 +-
>> mm/memory.c | 2 +-
>> mm/rmap.c | 2 +-
>> mm/swap_state.c | 2 +-
>> mm/swapfile.c | 287 +++++++++++++++++++++++++++++++++---------------
>> 6 files changed, 214 insertions(+), 93 deletions(-)
> I'm probably missing some background, but I find the patch hard to
> read. Can you disseminate some of this patch changelog into kernel-doc
> commentary so it's easier to follow which helpers do what relative to
> THP swap.

Yes. This is a good idea. Thanks for pointing it out. I will add more
kernel-doc commentary to make the code easier to be understood.

>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index d3bbf6bea9e9..213d32e57c39 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -80,6 +80,11 @@ extern struct kobj_attribute shmem_enabled_attr;
>> +static inline bool thp_swap_supported(void)
>> +{
>> +}
>> +
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index f73eafcaf4e9..57aa655ab27d 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -451,8 +451,8 @@ extern swp_entry_t get_swap_page_of_type(int);
>> extern int get_swap_pages(int n, bool cluster, swp_entry_t swp_entries[]);
>> extern int add_swap_count_continuation(swp_entry_t, gfp_t);
>> extern void swap_shmem_alloc(swp_entry_t);
>> -extern int swap_duplicate(swp_entry_t);
>> -extern int swapcache_prepare(swp_entry_t);
>> +extern int swap_duplicate(swp_entry_t *entry, bool cluster);
> This patch introduces a new flag to swap_duplicate(), but then all all
> usages still pass 'false' so why does this patch change the argument.
> Seems this change belongs to another patch?

This patch just introduce the capability to deal with huge swap entry in
swap_duplicate() family functions. The first user of the huge swap
entry is in

[PATCH -mm -v4 08/21] mm, THP, swap: Support to read a huge swap cluster for swapin a THP

via swapcache_prepare().

Yes, it is generally better to put the implementation and the user into
one patch. But I found in that way, I have to put most code of this
patchset into single huge patch, that is not good for review too. So I
made some compromise to separate the implementation and the users into
different patches to make the size of single patch not too huge. Does
this make sense to you?

>> +extern int swapcache_prepare(swp_entry_t entry, bool cluster);
> Rather than add a cluster flag to these helpers can the swp_entry_t
> carry the cluster flag directly?

Matthew Wilcox suggested to replace the "cluster" flag with the number
of entries to make the interface more flexible. And he suggest to use a
very smart way to encode the nr_entries into swap_entry_t with something

But I think we need to

- encode swap type, swap offset, nr_entries into a new swp_entry_t
- call a function
- decode the new swp_entry_t in the function

So it appears that it doesn't bring real value except reduce one
parameter. Or you suggest something else?

Best Regards,
Huang, Ying

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