Dear Jonathan,
> Here's a quick review from a non-expert (i.e., me). ...
Thanks!
> ... 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.
I was hoping that Ben would take that on; but OK, will do (first waiting
for maybe Ben's comments, and your reply to my questions below).
>> 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.
In a sense, I am seeking comments on why lowmem is ever polluted with
caches and why slabs are not cleared up without dropping pagecache, as
I feel that the "real bug" is lurking there. - I admit that I do not
understand MM, it seems a mess to my un-educatedf eyes. - Some of those
comments are questions, some others I am sure are bugs.
OK, will use [RFC/PATCH] in the subject.
>> +/* 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).
I added one new call, so I could easily call it from elsewhere without
having to (cross?)-declare many things; in fact would have preferred to
call iterate_supers() and drop_slab() directly.
Yes, easy_drop_caches() might be declared in some include file, but
should that be <linux/mm.h> or <linux/fs.h>? Would add another file to
those changed, seemed more direct this way.
Should I change something?
>> --- 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?
Yes, it was one of the unimportant oddities I noticed.
> [...]
>> @@ -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.
Yes, I noticed some precedents.
> [...]
>> + /*
>> + * 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>.
That made my head spin... Thanks for the pointers, will go through them
sometime.
>> + /* Subtract min_free_kbytes */
>> + if (min_free_kbytes > 0)
>> + y = min_free_kbytes >> (PAGE_SHIFT - 10);
>
> Why the "if"?
Sanity-check paranoia: because I do not trust the input values. Maybe
should have
BUG_ON(min_free_kbytes < 0);
but that does not belong there.
> 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);
Do you think I should change my patch? To me, it seemed clearer the way
I had it.
> [...]
>> @@ -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?
I guess because most other such things are also unsigned long; and I
guess they were so when they meant memory addresses. These are now page
counts, cannot overflow (with current limit of 64GB RAM).
>> @@ -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.
Yes. On one hand the code sometimes pauses for HZ/10 or so, and on the
other hand we have this routine working so hard to return the minimum
possible value.
>> @@ -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.
Will try to look it up sometime. - I had printk() to tell me the value
of pause, and mostly I got zero. Wonder how others measured it.
> [...]
>> --- 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?
Hmm... I would much prefer to leave it as is.
Thanks, Paul
Paul Szabo p...@maths.usyd.edu.au http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics University of Sydney Australia
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