From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f72.google.com (mail-pl0-f72.google.com [209.85.160.72]) by kanga.kvack.org (Postfix) with ESMTP id 9D4796B0273 for ; Mon, 9 Jul 2018 03:38:56 -0400 (EDT) Received: by mail-pl0-f72.google.com with SMTP id t19-v6so9548224plo.9 for ; Mon, 09 Jul 2018 00:38:56 -0700 (PDT) Received: from mga01.intel.com (mga01.intel.com. [192.55.52.88]) by mx.google.com with ESMTPS id d5-v6si14586032pla.337.2018.07.09.00.38.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 09 Jul 2018 00:38:54 -0700 (PDT) From: "Huang\, Ying" Subject: Re: [PATCH -mm -v4 03/21] mm, THP, swap: Support PMD swap mapping in swap_duplicate() References: <20180622035151.6676-1-ying.huang@intel.com> <20180622035151.6676-4-ying.huang@intel.com> Date: Mon, 09 Jul 2018 15:38:39 +0800 In-Reply-To: (Dan Williams's message of "Sat, 7 Jul 2018 16:22:54 -0700") Message-ID: <8736wsluyo.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: Andrew Morton , linux-mm , Linux Kernel Mailing List , "Kirill A. Shutemov" , Andrea Arcangeli , Michal Hocko , Johannes Weiner , Shaohua Li , hughd@google.com, Minchan Kim , Rik van Riel , Dave Hansen , n-horiguchi@ah.jp.nec.com, zi.yan@cs.rutgers.edu, daniel.m.jordan@oracle.com 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; >> #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT) >> #define HPAGE_PMD_NR (1<> >> +static inline bool thp_swap_supported(void) >> +{ >> + return IS_ENABLED(CONFIG_THP_SWAP); >> +} >> + >> #ifdef CONFIG_TRANSPARENT_HUGEPAGE >> #define HPAGE_PMD_SHIFT PMD_SHIFT >> #define HPAGE_PMD_SIZE ((1UL) << HPAGE_PMD_SHIFT) >> 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 like, https://plus.google.com/117536210417097546339/posts/hvctn17WUZu 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