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...
>
> ...
>
> --- 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_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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