Re: [PATCH] Revert "mm/page-writeback.c: print a warning if the vm dirtiness settings are illogical" (was: Re: [PATCH] mm: print a warning once the vm dirtiness settings is illogical)

[Jan Kara <jack@suse.cz>]

Hi Yafang,

On Tue 28-11-17 11:11:40, Yafang Shao wrote:
> What about bellow change ?
> It makes the function  domain_dirty_limits() more clear.
> And the result will have a higher precision.

Frankly, I don't find this any better and you've just lost the additional
precision of ratios computed in the "if (gdtc)" branch the multiplication by
PAGE_SIZE got us.

								Honza


> 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@suse.com>:
> > 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@suse.com>
> > 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@I-love.SAKURA.ne.jp>
> > Debugged-by: Yafang Shao <laoar.shao@gmail.com>
> > Fixes: 0f6d24f87856 ("mm/page-writeback.c: print a warning if the vm dirtiness settings are illogical")
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > ---
> >  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
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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