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SubjectRe: [RFC][PATCH 08/13] mm/migrate: demote pages during reclaim
On Mon, Jan 25, 2021 at 4:41 PM Dave Hansen <dave.hansen@linux.intel.com> wrote:
>
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> This is mostly derived from a patch from Yang Shi:
>
> https://lore.kernel.org/linux-mm/1560468577-101178-10-git-send-email-yang.shi@linux.alibaba.com/
>
> Add code to the reclaim path (shrink_page_list()) to "demote" data
> to another NUMA node instead of discarding the data. This always
> avoids the cost of I/O needed to read the page back in and sometimes
> avoids the writeout cost when the pagee is dirty.
>
> A second pass through shrink_page_list() will be made if any demotions
> fail. This essentally falls back to normal reclaim behavior in the
> case that demotions fail. Previous versions of this patch may have
> simply failed to reclaim pages which were eligible for demotion but
> were unable to be demoted in practice.
>
> Note: This just adds the start of infratructure for migration. It is
> actually disabled next to the FIXME in migrate_demote_page_ok().
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: osalvador <osalvador@suse.de>
>
> --
>
> changes from 202010:
> * add MR_NUMA_MISPLACED to trace MIGRATE_REASON define
> * make migrate_demote_page_ok() static, remove 'sc' arg until
> later patch
> * remove unnecessary alloc_demote_page() hugetlb warning
> * Simplify alloc_demote_page() gfp mask. Depend on
> __GFP_NORETRY to make it lightweight instead of fancier
> stuff like leaving out __GFP_IO/FS.
> * Allocate migration page with alloc_migration_target()
> instead of allocating directly.
> changes from 20200730:
> * Add another pass through shrink_page_list() when demotion
> fails.
> ---
>
> b/include/linux/migrate.h | 9 ++++
> b/include/trace/events/migrate.h | 3 -
> b/mm/vmscan.c | 81 +++++++++++++++++++++++++++++++++++++++
> 3 files changed, 92 insertions(+), 1 deletion(-)
>
> diff -puN include/linux/migrate.h~demote-with-migrate_pages include/linux/migrate.h
> --- a/include/linux/migrate.h~demote-with-migrate_pages 2021-01-25 16:23:14.591866696 -0800
> +++ b/include/linux/migrate.h 2021-01-25 16:23:14.599866696 -0800
> @@ -27,6 +27,7 @@ enum migrate_reason {
> MR_MEMPOLICY_MBIND,
> MR_NUMA_MISPLACED,
> MR_CONTIG_RANGE,
> + MR_DEMOTION,
> MR_TYPES
> };
>
> @@ -196,6 +197,14 @@ struct migrate_vma {
> int migrate_vma_setup(struct migrate_vma *args);
> void migrate_vma_pages(struct migrate_vma *migrate);
> void migrate_vma_finalize(struct migrate_vma *migrate);
> +int next_demotion_node(int node);
> +
> +#else /* CONFIG_MIGRATION disabled: */
> +
> +static inline int next_demotion_node(int node)
> +{
> + return NUMA_NO_NODE;
> +}
>
> #endif /* CONFIG_MIGRATION */
>
> diff -puN include/trace/events/migrate.h~demote-with-migrate_pages include/trace/events/migrate.h
> --- a/include/trace/events/migrate.h~demote-with-migrate_pages 2021-01-25 16:23:14.593866696 -0800
> +++ b/include/trace/events/migrate.h 2021-01-25 16:23:14.599866696 -0800
> @@ -20,7 +20,8 @@
> EM( MR_SYSCALL, "syscall_or_cpuset") \
> EM( MR_MEMPOLICY_MBIND, "mempolicy_mbind") \
> EM( MR_NUMA_MISPLACED, "numa_misplaced") \
> - EMe(MR_CONTIG_RANGE, "contig_range")
> + EM( MR_CONTIG_RANGE, "contig_range") \
> + EMe(MR_DEMOTION, "demotion")
>
> /*
> * First define the enums in the above macros to be exported to userspace
> diff -puN mm/vmscan.c~demote-with-migrate_pages mm/vmscan.c
> --- a/mm/vmscan.c~demote-with-migrate_pages 2021-01-25 16:23:14.595866696 -0800
> +++ b/mm/vmscan.c 2021-01-25 16:23:14.601866696 -0800
> @@ -43,6 +43,7 @@
> #include <linux/kthread.h>
> #include <linux/freezer.h>
> #include <linux/memcontrol.h>
> +#include <linux/migrate.h>
> #include <linux/delayacct.h>
> #include <linux/sysctl.h>
> #include <linux/oom.h>
> @@ -1036,6 +1037,24 @@ static enum page_references page_check_r
> return PAGEREF_RECLAIM;
> }
>
> +static bool migrate_demote_page_ok(struct page *page)
> +{
> + int next_nid = next_demotion_node(page_to_nid(page));
> +
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + VM_BUG_ON_PAGE(PageHuge(page), page);
> + VM_BUG_ON_PAGE(PageLRU(page), page);
> +
> + if (next_nid == NUMA_NO_NODE)
> + return false;
> + if (PageTransHuge(page) && !thp_migration_supported())
> + return false;
> +
> + // FIXME: actually enable this later in the series
> + return false;
> +}
> +
> +
> /* Check if a page is dirty or under writeback */
> static void page_check_dirty_writeback(struct page *page,
> bool *dirty, bool *writeback)
> @@ -1066,6 +1085,44 @@ static void page_check_dirty_writeback(s
> mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
> }
>
> +static struct page *alloc_demote_page(struct page *page, unsigned long node)
> +{
> + struct migration_target_control mtc = {
> + /*
> + * Fail quickly and quietly. Page will likely
> + * just be discarded instead of migrated.
> + */
> + .gfp_mask = GFP_HIGHUSER | __GFP_NORETRY | __GFP_NOWARN,
> + .nid = node
> + };
> +
> + return alloc_migration_target(page, (unsigned long)&mtc);

Other than the gfp flag question raised by Oscar, I'm wondering how we
guarantee the demotion allocation happens on the designated node. In
the previous version __GFP_THISNODE is set to guarantee this. In this
version you switched to use alloc_migration_target() API but without
having nodemask or __GFP_THISNODE. If nodemask is NULL the allocation
may fall back to an unexpected node.

And GFP_HIGHUSER does respect cpuset, so if the demotion target node
is excluded by the cpuset which the task belongs to, the migration
would fail. This might be a way to respect cpuset, but it should just
work for direct reclaimer. So, is this change really expected?

> +}
> +
> +/*
> + * Take pages on @demote_list and attempt to demote them to
> + * another node. Pages which are not demoted are left on
> + * @demote_pages.
> + */
> +static unsigned int demote_page_list(struct list_head *demote_pages,
> + struct pglist_data *pgdat,
> + struct scan_control *sc)
> +{
> + int target_nid = next_demotion_node(pgdat->node_id);
> + unsigned int nr_succeeded = 0;
> + int err;
> +
> + if (list_empty(demote_pages))
> + return 0;
> +
> + /* Demotion ignores all cpuset and mempolicy settings */
> + err = migrate_pages(demote_pages, alloc_demote_page, NULL,
> + target_nid, MIGRATE_ASYNC, MR_DEMOTION,
> + &nr_succeeded);
> +
> + return nr_succeeded;
> +}
> +
> /*
> * shrink_page_list() returns the number of reclaimed pages
> */
> @@ -1078,12 +1135,15 @@ static unsigned int shrink_page_list(str
> {
> LIST_HEAD(ret_pages);
> LIST_HEAD(free_pages);
> + LIST_HEAD(demote_pages);
> unsigned int nr_reclaimed = 0;
> unsigned int pgactivate = 0;
> + bool do_demote_pass = true;
>
> memset(stat, 0, sizeof(*stat));
> cond_resched();
>
> +retry:
> while (!list_empty(page_list)) {
> struct address_space *mapping;
> struct page *page;
> @@ -1233,6 +1293,16 @@ static unsigned int shrink_page_list(str
> }
>
> /*
> + * Before reclaiming the page, try to relocate
> + * its contents to another node.
> + */
> + if (do_demote_pass && migrate_demote_page_ok(page)) {
> + list_add(&page->lru, &demote_pages);
> + unlock_page(page);
> + continue;
> + }
> +
> + /*
> * Anonymous process memory has backing store?
> * Try to allocate it some swap space here.
> * Lazyfree page could be freed directly
> @@ -1479,6 +1549,17 @@ keep:
> list_add(&page->lru, &ret_pages);
> VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
> }
> + /* 'page_list' is always empty here */
> +
> + /* Migrate pages selected for demotion */
> + nr_reclaimed += demote_page_list(&demote_pages, pgdat, sc);
> + /* Pages that could not be demoted are still in @demote_pages */
> + if (!list_empty(&demote_pages)) {
> + /* Pages which failed to demoted go back on on @page_list for retry: */
> + list_splice_init(&demote_pages, page_list);
> + do_demote_pass = false;
> + goto retry;
> + }
>
> pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
>
> _
>

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