Hi Paul,
Paul Szabo wrote:
> I changed the proposed patch accordingly, scripts/checkpatch.pl produces
> just a few warnings. I had my patch in use for a while now, so I believe
> it is suitably tested.
Thanks much --- this is easier to read.
Here's a quick review from a non-expert (i.e., me). When the patch
seems ready to you, please send it inline in a message to
linux...@kvack.org, cc-ing linux-ker...@vger.kernel.org and Ben and me
(or this bug log) so we can track it. The mm folks will probably have
more feedback, and we can take it from there.
> Avoid OOM when filesystem caches fill lowmem and are not reclaimed,
> doing drop_caches at that point. The issue is easily reproducible on
> machines with over 32GB RAM. The patch correctly protects against OOM.
> The added call to drop_caches has been observed to trigger "needlessly"
> but on quite rare occasions only.
Sounds like a reasonable strategy.
> Also included are several minor fixes:
Those should go in separate patches, but if you're just seeking
comments then lumping them with the main change in a patch with
[RFC/PATCH] in the subject line is fine.
[...]
> Signed-off-by: Paul Szabo <p...@maths.usyd.edu.au>
Thanks.
> --- fs/drop_caches.c.old 2012-10-17 13:50:15.000000000 +1100
> +++ fs/drop_caches.c 2013-01-04 21:52:47.000000000 +1100
> @@ -65,3 +65,10 @@ int drop_caches_sysctl_handler(ctl_table
> }
> return 0;
> }
> +
> +/* Easy call: do "echo 3 > /proc/sys/vm/drop_caches" */
I don't understand this comment. What does "Easy call" mean?
> +void easy_drop_caches(void)
> +{
> + iterate_supers(drop_pagecache_sb, NULL);
> + drop_slab();
> +}
This should be declared in some appropriate header (e.g.,
include/linux/mm.h).
> --- mm/page-writeback.c.old 2012-10-17 13:50:15.000000000 +1100
> +++ mm/page-writeback.c 2013-01-06 21:54:59.000000000 +1100
> @@ -39,7 +39,8 @@
> /*
> * Sleep at most 200ms at a time in balance_dirty_pages().
> */
> -#define MAX_PAUSE max(HZ/5, 1)
> +/* Might as well be max(HZ/5,4) to ensure max_pause/4>0 always */
> +#define MAX_PAUSE max(HZ/5, 4)
This is one of the "while at it"s rather than the main point of
the patch, right?
[...]
> @@ -343,11 +344,26 @@ static unsigned long highmem_dirtyable_m
> unsigned long determine_dirtyable_memory(void)
> {
> unsigned long x;
> + int y = 0;
> + extern int min_free_kbytes;
Probably should be declared in a header like mm/internal.h, but
there's precedent for the ad-hoc declaration, so meh.
[...]
> + /*
> + * Seems that highmem_is_dirtyable is only used here, in the
> + * calculation of limits and threshholds of dirtiness, not in deciding
> + * where to put dirty things. Is that so? Is that as should be?
> + * What is the recommended setting of highmem_is_dirtyable?
> + */
> if (!vm_highmem_is_dirtyable)
> x -= highmem_dirtyable_memory(x);
I dunno. See 195cf453d2c3 (mm/page-writeback: highmem_is_dirtyable
option, 2008-02-04) and the thread surrounding
<http://thread.gmane.org/gmane.linux.alsa.devel/49438/focus=603793>.
> + /* Subtract min_free_kbytes */
> + if (min_free_kbytes > 0)
> + y = min_free_kbytes >> (PAGE_SHIFT - 10);
Why the "if"?
If I were doing it, I think I'd unconditionally set 'y' here, for
clarity and to avoid bugs if some later patch reuses the var for
something else.
y = min_free_kbytes >> (PAGE_SHIFT - 10);
x -= min(x, y);
[...]
> @@ -559,7 +578,7 @@ static unsigned long bdi_position_ratio(
> * => fast response on large errors; small oscillation near setpoint
> */
> setpoint = (freerun + limit) / 2;
> - x = div_s64((setpoint - dirty) << RATELIMIT_CALC_SHIFT,
> + x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> limit - setpoint + 1);
Why are 'setpoint' and 'dirty' of type unsigned long instead of long
in the first place? What happens if one of their values overflows a
long?
> pos_ratio = x;
> pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> @@ -995,6 +1014,13 @@ static unsigned long bdi_max_pause(struc
> * The pause time will be settled within range (max_pause/4, max_pause).
> * Apply a minimal value of 4 to get a non-zero max_pause/4.
> */
> + /*
> + * On large machine it seems we always return 4,
> + * on smaller desktop machine mostly return 5 (rarely 9 or 14).
> + * Are those too small? Should we return something fixed e.g.
> + return (HZ/10);
> + * instead of this wasted/useless calculation?
> + */
> return clamp_val(t, 4, MAX_PAUSE);
Another "while at it", I guess.
> }
>
> @@ -1109,6 +1135,11 @@ static void balance_dirty_pages(struct a
> }
> pause = HZ * pages_dirtied / task_ratelimit;
> if (unlikely(pause <= 0)) {
> + /*
> + * Not unlikely: often we get zero.
> + * Seems we always get 0 on large machine.
> + * Should not do a pause of 1 here?
> + */
> trace_balance_dirty_pages(bdi,
"git log -S'if (unlikely(pause <= 0))' -- mm/page-writeback.c" tells
me this is from 57fc978cfb61 (writeback: control dirty pause time,
2011-06-11), in case that helps.
[...]
> --- mm/vmscan.c.old 2012-10-17 13:50:15.000000000 +1100
> +++ mm/vmscan.c 2013-01-06 09:50:49.000000000 +1100
[...]
> @@ -2726,9 +2731,87 @@ loop_again:
> nr_slab = shrink_slab(&shrink, sc.nr_scanned,
> lru_pages);
> sc.nr_reclaimed +=
> reclaim_state->reclaimed_slab;
> total_scanned += sc.nr_scanned;
> + if (unlikely(
> + i == 1 &&
> + nr_slab < 10 &&
> + (reclaim_state->reclaimed_slab) < 10 &&
> + zone_page_state(zone, NR_SLAB_RECLAIMABLE)
> > 10 &&
> + !zone_watermark_ok_safe(zone, order,
> + high_wmark_pages(zone),
> end_zone, 0))) {
> + /*
> + * We are stressed (desperate), better
This is getting really deeply nested. Would it be possible to split
out a function so this code could be more easily contemplated in
isolation?
Thanks for taking this on and hope that helps.
Regards,
Jonathan
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