On 4/2/20 12:57 AM, Pavel Tatashin wrote:
> Initializing struct pages is a long task and keeping interrupts disabled
> for the duration of this operation introduces a number of problems.
>
> 1. jiffies are not updated for long period of time, and thus incorrect time
> is reported. See proposed solution and discussion here:
> lkml/20200311123848.118638-1-shile.zhang@xxxxxxxxxxxxxxxxx
> 2. It prevents farther improving deferred page initialization by allowing
> intra-node multi-threading.
>
> We are keeping interrupts disabled to solve a rather theoretical problem
> that was never observed in real world (See 3a2d7fa8a3d5).
>
> Lets keep interrupts enabled. In case we ever encounter a scenario where
> an interrupt thread wants to allocate large amount of memory this early in
> boot we can deal with that by growing zone (see deferred_grow_zone()) by
> the needed amount before starting deferred_init_memmap() threads.
>
> Before:
> [ 1.232459] node 0 initialised, 12058412 pages in 1ms
>
> After:
> [ 1.632580] node 0 initialised, 12051227 pages in 436ms
>
> Fixes: 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred pages")
> Cc: stable@xxxxxxxxxxxxxxx # 4.17+
TBH I don't remember my concern anymore. Reading my mail now [1] it seems I was
thinking the problem could happen not just in interrupt context, but with other
kthreads as well.
Anyway I agree with the approach of waiting for actual issues being reported and
then eventually pre-growing more.
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
[1] https://lore.kernel.org/linux-mm/33e3a3ff-0318-1a07-3c57-6be638046c87@xxxxxxx/
> Reported-by: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> include/linux/mmzone.h | 2 ++
> mm/page_alloc.c | 22 ++++++++--------------
> 2 files changed, 10 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 462f6873905a..c5bdf55da034 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -721,6 +721,8 @@ typedef struct pglist_data {
> /*
> * Must be held any time you expect node_start_pfn,
> * node_present_pages, node_spanned_pages or nr_zones to stay constant.
> + * Also synchronizes pgdat->first_deferred_pfn during deferred page
> + * init.
> *
> * pgdat_resize_lock() and pgdat_resize_unlock() are provided to
> * manipulate node_size_lock without checking for CONFIG_MEMORY_HOTPLUG
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e8ff6a176164..68669d3a5a66 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1790,6 +1790,13 @@ static int __init deferred_init_memmap(void *data)
> BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
> pgdat->first_deferred_pfn = ULONG_MAX;
>
> + /*
> + * Once we unlock here, the zone cannot be grown anymore, thus if an
> + * interrupt thread must allocate this early in boot, zone must be
> + * pre-grown prior to start of deferred page initialization.
> + */
> + pgdat_resize_unlock(pgdat, &flags);
> +
> /* Only the highest zone is deferred so find it */
> for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> zone = pgdat->node_zones + zid;
> @@ -1809,11 +1816,9 @@ static int __init deferred_init_memmap(void *data)
> */
> while (spfn < epfn) {
> nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> - touch_nmi_watchdog();
> + cond_resched();
> }
> zone_empty:
> - pgdat_resize_unlock(pgdat, &flags);
> -
> /* Sanity check that the next zone really is unpopulated */
> WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));
>
> @@ -1855,17 +1860,6 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
>
> pgdat_resize_lock(pgdat, &flags);
>
> - /*
> - * If deferred pages have been initialized while we were waiting for
> - * the lock, return true, as the zone was grown. The caller will retry
> - * this zone. We won't return to this function since the caller also
> - * has this static branch.
> - */
> - if (!static_branch_unlikely(&deferred_pages)) {
> - pgdat_resize_unlock(pgdat, &flags);
> - return true;
> - }
> -
> /*
> * If someone grew this zone while we were waiting for spinlock, return
> * true, as there might be enough pages already.
>
