Re: [patch 2/4] mm: writeback: distribute write pages across allowable zones

Shaohua Li
2011-09-21T11:04:36Z

On Tue, 2011-09-20 at 21:45 +0800, Johannes Weiner wrote:
> This patch allows allocators to pass __GFP_WRITE when they know in
> advance that the allocated page will be written to and become dirty
> soon. The page allocator will then attempt to distribute those
> allocations across zones, such that no single zone will end up full of
> dirty, and thus more or less, unreclaimable pages.
>
> The global dirty limits are put in proportion to the respective zone's
> amount of dirtyable memory and allocations diverted to other zones
> when the limit is reached.
>
> For now, the problem remains for NUMA configurations where the zones
> allowed for allocation are in sum not big enough to trigger the global
> dirty limits, but a future approach to solve this can reuse the
> per-zone dirty limit infrastructure laid out in this patch to have
> dirty throttling and the flusher threads consider individual zones.
>
> Signed-off-by: Johannes Weiner <jweiner@redhat.com>
> ---
> include/linux/gfp.h | 4 ++-
> include/linux/writeback.h | 1 +
> mm/page-writeback.c | 66 +++++++++++++++++++++++++++++++++++++-------
> mm/page_alloc.c | 22 ++++++++++++++-
> 4 files changed, 80 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 3a76faf..50efc7e 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -36,6 +36,7 @@ struct vm_area_struct;
> #endif
> #define ___GFP_NO_KSWAPD 0x400000u
> #define ___GFP_OTHER_NODE 0x800000u
> +#define ___GFP_WRITE 0x1000000u
>
> /*
> * GFP bitmasks..
> @@ -85,6 +86,7 @@ struct vm_area_struct;
>
> #define __GFP_NO_KSWAPD ((__force gfp_t)___GFP_NO_KSWAPD)
> #define __GFP_OTHER_NODE ((__force gfp_t)___GFP_OTHER_NODE) /* On behalf of other node */
> +#define __GFP_WRITE ((__force gfp_t)___GFP_WRITE) /* Allocator intends to dirty page */
>
> /*
> * This may seem redundant, but it's a way of annotating false positives vs.
> @@ -92,7 +94,7 @@ struct vm_area_struct;
> */
> #define __GFP_NOTRACK_FALSE_POSITIVE (__GFP_NOTRACK)
>
> -#define __GFP_BITS_SHIFT 24 /* Room for N __GFP_FOO bits */
> +#define __GFP_BITS_SHIFT 25 /* Room for N __GFP_FOO bits */
> #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>
> /* This equals 0, but use constants in case they ever change */
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index a5f495f..c96ee0c 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -104,6 +104,7 @@ void laptop_mode_timer_fn(unsigned long data);
> static inline void laptop_sync_completion(void) { }
> #endif
> void throttle_vm_writeout(gfp_t gfp_mask);
> +bool zone_dirty_ok(struct zone *zone);
>
> extern unsigned long global_dirty_limit;
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 9f896db..1fc714c 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -142,6 +142,22 @@ unsigned long global_dirty_limit;
> static struct prop_descriptor vm_completions;
> static struct prop_descriptor vm_dirties;
>
> +static unsigned long zone_dirtyable_memory(struct zone *zone)
> +{
> + unsigned long x;
> + /*
> + * To keep a reasonable ratio between dirty memory and lowmem,
> + * highmem is not considered dirtyable on a global level.
> + *
> + * But we allow individual highmem zones to hold a potentially
> + * bigger share of that global amount of dirty pages as long
> + * as they have enough free or reclaimable pages around.
> + */
> + x = zone_page_state(zone, NR_FREE_PAGES) - zone->totalreserve_pages;
> + x += zone_reclaimable_pages(zone);
> + return x;
> +}
> +
> /*
> * Work out the current dirty-memory clamping and background writeout
> * thresholds.
> @@ -417,7 +433,7 @@ static unsigned long hard_dirty_limit(unsigned long thresh)
> }
>
> /*
> - * global_dirty_limits - background-writeback and dirty-throttling thresholds
> + * dirty_limits - background-writeback and dirty-throttling thresholds
> *
> * Calculate the dirty thresholds based on sysctl parameters
> * - vm.dirty_background_ratio or vm.dirty_background_bytes
> @@ -425,24 +441,35 @@ static unsigned long hard_dirty_limit(unsigned long thresh)
> * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
> * real-time tasks.
> */
> -void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
> +static void dirty_limits(struct zone *zone,
> + unsigned long *pbackground,
> + unsigned long *pdirty)
> {
> + unsigned long uninitialized_var(zone_memory);
> + unsigned long available_memory;
> + unsigned long global_memory;
> unsigned long background;
> - unsigned long dirty;
> - unsigned long uninitialized_var(available_memory);
> struct task_struct *tsk;
> + unsigned long dirty;
>
> - if (!vm_dirty_bytes || !dirty_background_bytes)
> - available_memory = determine_dirtyable_memory();
> + global_memory = determine_dirtyable_memory();
> + if (zone)
> + available_memory = zone_memory = zone_dirtyable_memory(zone);
> + else
> + available_memory = global_memory;
>
> - if (vm_dirty_bytes)
> + if (vm_dirty_bytes) {
> dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
> - else
> + if (zone)
> + dirty = dirty * zone_memory / global_memory;
> + } else
> dirty = (vm_dirty_ratio * available_memory) / 100;
>
> - if (dirty_background_bytes)
> + if (dirty_background_bytes) {
> background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
> - else
> + if (zone)
> + background = background * zone_memory / global_memory;
> + } else
> background = (dirty_background_ratio * available_memory) / 100;
>
> if (background >= dirty)
> @@ -452,9 +479,15 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
> background += background / 4;
> dirty += dirty / 4;
> }
> + if (!zone)
> + trace_global_dirty_state(background, dirty);
> *pbackground = background;
> *pdirty = dirty;
> - trace_global_dirty_state(background, dirty);
> +}
> +
> +void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
> +{
> + dirty_limits(NULL, pbackground, pdirty);
> }
>
> /**
> @@ -875,6 +908,17 @@ void throttle_vm_writeout(gfp_t gfp_mask)
> }
> }
>
> +bool zone_dirty_ok(struct zone *zone)
> +{
> + unsigned long background_thresh, dirty_thresh;
> +
> + dirty_limits(zone, &background_thresh, &dirty_thresh);
> +
> + return zone_page_state(zone, NR_FILE_DIRTY) +
> + zone_page_state(zone, NR_UNSTABLE_NFS) +
> + zone_page_state(zone, NR_WRITEBACK) <= dirty_thresh;
> +}
> +
> /*
> * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
> */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7e8e2ee..3cca043 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1368,6 +1368,7 @@ failed:
> #define ALLOC_HARDER 0x10 /* try to alloc harder */
> #define ALLOC_HIGH 0x20 /* __GFP_HIGH set */
> #define ALLOC_CPUSET 0x40 /* check for correct cpuset */
> +#define ALLOC_SLOWPATH 0x80 /* allocator retrying */
>
> #ifdef CONFIG_FAIL_PAGE_ALLOC
>
> @@ -1667,6 +1668,25 @@ zonelist_scan:
> if ((alloc_flags & ALLOC_CPUSET) &&
> !cpuset_zone_allowed_softwall(zone, gfp_mask))
> continue;
> + /*
> + * This may look like it would increase pressure on
> + * lower zones by failing allocations in higher zones
> + * before they are full. But once they are full, the
> + * allocations fall back to lower zones anyway, and
> + * then this check actually protects the lower zones
> + * from a flood of dirty page allocations.
if increasing pressure on lower zones isn't a problem since higher zones
will eventually be full, how about a workload without too many writes,
so higher zones will not be full. In such case, increasing low zone
pressure sounds not good.

Thanks,
Shaohua