Re: [PATCH 16/16] mm: strictlimit feature

From: Maxim Patlasov
Date: Tue Jul 02 2013 - 04:34:17 EST

Hi Andrew,

07/02/2013 01:16 AM, Andrew Morton ÐÐÑÐÑ:
On Sat, 29 Jun 2013 21:48:54 +0400 Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx> wrote:

From: Miklos Szeredi <mszeredi@xxxxxxx>

The feature prevents mistrusted filesystems to grow a large number of dirty
pages before throttling. For such filesystems balance_dirty_pages always
check bdi counters against bdi limits. I.e. even if global "nr_dirty" is under
"freerun", it's not allowed to skip bdi checks. The only use case for now is
fuse: it sets bdi max_ratio to 1% by default and system administrators are
supposed to expect that this limit won't be exceeded.

The feature is on if address space is marked by AS_STRICTLIMIT flag.
A filesystem may set the flag when it initializes a new inode.

Fengguang, could you please review this patch?

I suggest you await the next version, which hopefully will be more
reviewable...

Thanks a lot for quick review, I'll update the patch according to your comments soon.

I'm answering the question about BDI_idle below inline, but I'll add some comment about it where BDI_idle is actually used as well.

Thanks,
Maxim


...

--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -33,6 +33,8 @@ enum bdi_state {
BDI_sync_congested, /* The sync queue is getting full */
BDI_registered, /* bdi_register() was done */
BDI_writeback_running, /* Writeback is in progress */
+ BDI_idle, /* No pages under writeback at the moment of
+ * last update of write bw */
Why does BDI_idle exist?

BDI_idle along with BDI_WRITTEN_BACK exists to distinguish two cases:

1st. BDI_WRITTEN has not been incremented since we looked at it last time because backing dev is unresponding. I.e. it had some pages under writeback but it have not made any progress for some reasons.

2nd. BDI_WRITTEN has not been incremented since we looked at it last time because backing dev had nothing to do. I.e. there are some dirty pages on bdi, but they have not been passed to backing dev yet. This is the case when bdi_dirty is under bdi background threshold and flusher refrains from flushing even if we woke it up explicitly by bdi_start_background_writeback.

We have to skip bdi_update_write_bandwidth() in the 2nd case because otherwise bdi_update_write_bandwidth() will see written==0 and mistakenly decrease write_bandwidth. The criterion to skip is the following: BDI_idle is set (i.e. there were no pages under writeback when we looked at the bdi last time) && BDI_WRITTEN_BACK counter has not changed (i.e. no new pages has been sent to writeback since we looked at the bdi last time).

Thanks,
Maxim


BDI_unused, /* Available bits start here */
};
@@ -43,6 +45,7 @@ enum bdi_stat_item {
BDI_WRITEBACK,
BDI_DIRTIED,
BDI_WRITTEN,
+ BDI_WRITTEN_BACK,
NR_BDI_STAT_ITEMS
};
@@ -76,6 +79,8 @@ struct backing_dev_info {
unsigned long bw_time_stamp; /* last time write bw is updated */
unsigned long dirtied_stamp;
unsigned long written_stamp; /* pages written at bw_time_stamp */
+ unsigned long writeback_stamp; /* pages sent to writeback at
+ * bw_time_stamp */
Well this sucks. Some of the "foo_stamp" fields are in units of time
(jiffies? We aren't told) and some of the "foo_stamp" fields are in
units of number-of-pages. It would be good to fix the naming here.

unsigned long write_bandwidth; /* the estimated write bandwidth */
unsigned long avg_write_bandwidth; /* further smoothed write bw */
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e3dea75..baac702 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -25,6 +25,7 @@ enum mapping_flags {
AS_MM_ALL_LOCKS = __GFP_BITS_SHIFT + 2, /* under mm_take_all_locks() */
AS_UNEVICTABLE = __GFP_BITS_SHIFT + 3, /* e.g., ramdisk, SHM_LOCK */
AS_BALLOON_MAP = __GFP_BITS_SHIFT + 4, /* balloon page special map */
+ AS_STRICTLIMIT = __GFP_BITS_SHIFT + 5, /* strict dirty limit */
Thing is, "strict dirty limit" isn't documented anywhere, so this
reference is left dangling.

...

--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -94,6 +94,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
"BackgroundThresh: %10lu kB\n"
"BdiDirtied: %10lu kB\n"
"BdiWritten: %10lu kB\n"
+ "BdiWrittenBack: %10lu kB\n"
"BdiWriteBandwidth: %10lu kBps\n"
"b_dirty: %10lu\n"
"b_io: %10lu\n"
I can't imagine what the difference is between BdiWritten and
BdiWrittenBack.

I suggest you document this at the BDI_WRITTEN_BACK definition site in
enum bdi_stat_item. BDI_WRITTEN (at least) will also need
documentation so people can understand the difference.

...

@@ -679,29 +711,31 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
if (unlikely(dirty >= limit))
return 0;
+ if (unlikely(strictlimit)) {
+ if (bdi_dirty < 8)
+ return 2 << RATELIMIT_CALC_SHIFT;
+
+ if (bdi_dirty >= bdi_thresh)
+ return 0;
+
+ bdi_setpoint = bdi_thresh + bdi_dirty_limit(bdi, bg_thresh);
+ bdi_setpoint /= 2;
+
+ if (bdi_setpoint == 0 || bdi_setpoint == bdi_thresh)
+ return 0;
+
+ pos_ratio = pos_ratio_polynom(bdi_setpoint, bdi_dirty,
+ bdi_thresh);
+ return min_t(long long, pos_ratio, 2 << RATELIMIT_CALC_SHIFT);
+ }
This would be a suitable site at which to document the strictlimit
feature. What it is, how it works and most importantly, why it exists.

...

@@ -994,6 +1029,16 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
* keep that period small to reduce time lags).
*/
step = 0;
+
+ if (unlikely(strictlimit)) {
+ dirty = bdi_dirty;
+ if (bdi_dirty < 8)
+ setpoint = bdi_dirty + 1;
+ else
+ setpoint = (bdi_thresh +
+ bdi_dirty_limit(bdi, bg_thresh)) / 2;
+ }
Explain this to the reader, please.

...




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