Hi Michal,
What about bellow change ?
It makes the function domain_dirty_limits() more clear.
And the result will have a higher precision.
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 8a15511..2b5e507 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -397,8 +397,8 @@ static void domain_dirty_limits(struct
dirty_throttle_control *dtc)
unsigned long bytes = vm_dirty_bytes;
unsigned long bg_bytes = dirty_background_bytes;
/* convert ratios to per-PAGE_SIZE for higher precision */
- unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
- unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
+ unsigned long ratio = vm_dirty_ratio;
+ unsigned long bg_ratio = dirty_background_ratio;
unsigned long thresh;
unsigned long bg_thresh;
struct task_struct *tsk;
@@ -416,28 +416,33 @@ static void domain_dirty_limits(struct
dirty_throttle_control *dtc)
*/
if (bytes)
ratio = min(DIV_ROUND_UP(bytes, global_avail),
- PAGE_SIZE);
+ 100);
if (bg_bytes)
bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
- PAGE_SIZE);
+ 99); /* bg_ratio should less than ratio */
bytes = bg_bytes = 0;
}
+ /* bytes and bg_bytes must be PAGE_SIZE aligned */
if (bytes)
- thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
+ thresh = DIV_ROUND_UP(bytes, PAGE_SIZE) * 100;
else
- thresh = (ratio * available_memory) / PAGE_SIZE;
+ thresh = ratio * available_memory;
if (bg_bytes)
- bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
+ bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE) * 100;
else
- bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
+ bg_thresh = bg_ratio * available_memory;
if (unlikely(bg_thresh >= thresh)) {
pr_warn("vm direct limit must be set greater than background limit.\n");
bg_thresh = thresh / 2;
}
+ /* ensure bg_thresh and thresh never be 0 */
+ bg_thresh = DIV_ROUND_UP(bg_thresh, 100);
+ thresh = DIV_ROUND_UP(thresh, 100);
+
tsk = current;
if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
2017-11-27 17:19 GMT+08:00 Michal Hocko <mhocko@xxxxxxxx>:
> Andrew,
> could you simply send this to Linus. If we _really_ need something to
> prevent misconfiguration, which I doubt to be honest, then it should be
> thought through much better.
> ---
> From 4ef6b1cbf98ea5dae155ab3303c4ae1d93411b79 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxxx>
> Date: Mon, 27 Nov 2017 10:12:15 +0100
> Subject: [PATCH] Revert "mm/page-writeback.c: print a warning if the vm
> dirtiness settings are illogical"
>
> This reverts commit 0f6d24f878568fac579a1962d0bf7cb9f01e0ceb because
> it causes false positive warnings during OOM situations as noticed by
> Tetsuo Handa:
> [ 621.814512] Node 0 active_anon:3525940kB inactive_anon:8372kB active_file:216kB inactive_file:1872kB unevictable:0kB isolated(anon):0kB isolated(file):0kB mapped:2504kB dirty:52kB writeback:0kB shmem:8660kB s
> hmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 636928kB writeback_tmp:0kB unstable:0kB all_unreclaimable? yes
> [ 621.821534] Node 0 DMA free:14848kB min:284kB low:352kB high:420kB active_anon:992kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:15988kB managed:15904kB mlocke
> d:0kB kernel_stack:0kB pagetables:24kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
> [ 621.829035] lowmem_reserve[]: 0 2687 3645 3645
> [ 621.831655] Node 0 DMA32 free:53004kB min:49608kB low:62008kB high:74408kB active_anon:2712648kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:3129216kB managed:
> 2773132kB mlocked:0kB kernel_stack:96kB pagetables:5096kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
> [ 621.839945] lowmem_reserve[]: 0 0 958 958
> [ 621.842811] Node 0 Normal free:17140kB min:17684kB low:22104kB high:26524kB active_anon:812300kB inactive_anon:8372kB active_file:1228kB inactive_file:1868kB unevictable:0kB writepending:52kB present:1048576k
> B managed:981224kB mlocked:0kB kernel_stack:3520kB pagetables:8552kB bounce:0kB free_pcp:120kB local_pcp:120kB free_cma:0kB
> [ 621.852473] lowmem_reserve[]: 0 0 0 0
> [...]
> [ 621.891477] Out of memory: Kill process 8459 (a.out) score 999 or sacrifice child
> [ 621.894363] Killed process 8459 (a.out) total-vm:4180kB, anon-rss:88kB, file-rss:0kB, shmem-rss:0kB
> [ 621.897172] oom_reaper: reaped process 8459 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> [ 622.424664] vm direct limit must be set greater than background limit.
>
> The problem is that both thresh and bg_thresh will be 0 if available_memory
> is less than 4 pages when evaluating global_dirtyable_memory. While
> this might be worked around the whole point of the warning is dubious at
> best. We do rely on admins to do sensible things when changing tunable
> knobs. Dirty memory writeback knobs are not any special in that regards
> so revert the warning rather than adding more hacks to work this around.
>
> Rerported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Debugged-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> Fixes: 0f6d24f87856 ("mm/page-writeback.c: print a warning if the vm dirtiness settings are illogical")
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> Documentation/sysctl/vm.txt | 7 -------
> mm/page-writeback.c | 5 +----
> 2 files changed, 1 insertion(+), 11 deletions(-)
>
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index b920423f88cb..5025ff9307e6 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -158,10 +158,6 @@ Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
> value lower than this limit will be ignored and the old configuration will be
> retained.
>
> -Note: the value of dirty_bytes also must be set greater than
> -dirty_background_bytes or the amount of memory corresponding to
> -dirty_background_ratio.
> -
> ==============================================================
>
> dirty_expire_centisecs
> @@ -181,9 +177,6 @@ generating disk writes will itself start writing out dirty data.
>
> The total available memory is not equal to total system memory.
>
> -Note: dirty_ratio must be set greater than dirty_background_ratio or
> -ratio corresponding to dirty_background_bytes.
> -
> ==============================================================
>
> dirty_writeback_centisecs
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index e7095030aa1f..586f31261c83 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -433,11 +433,8 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
> else
> bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
>
> - if (unlikely(bg_thresh >= thresh)) {
> - pr_warn("vm direct limit must be set greater than background limit.\n");
> + if (bg_thresh >= thresh)
> bg_thresh = thresh / 2;
> - }
> -
> tsk = current;
> if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
> bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
> --
> 2.15.0
>
> --
> Michal Hocko
> SUSE Labs
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