Re: [PATCH] zram: Prevent page allocation failure during zcomp_strm_alloc

From: Sergey Senozhatsky
Date: Thu Nov 19 2015 - 19:48:12 EST


On (11/19/15 22:49), wrote:
> I know what you mean (streams are not free).
> First of all, I'm sorry I would have to tell you why I try this patch.

nothing to be sorry about.

> When we're using LZ4 multi stream for zram swap, I found out this
> allocation failure message in system running test,
> That was not only once, but a few. Also, some failure cases were
> continually occurring to try allocation order 3.
> I believed that status makes time delay issue to process launch.

ahhh, I see. repetitive allocation failures during heavy I/O and low
memory conditions. hm... does it actually make any sense to make
something like this (*just an idea*)


diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 5cb13ca..049ae6b 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -124,6 +124,19 @@ static struct zcomp_strm *zcomp_strm_multi_find(struct zcomp *comp)
zstrm = zcomp_strm_alloc(comp);
if (!zstrm) {
+ /*
+ * If current IO path has failed to allocate a new
+ * stream then most likely lots of subsequent IO
+ * requests will do the same, wasting time attempting
+ * to reclaim pages, printing warning, etc. Reduce
+ * the number of max_stream and print an error.
+ */
+ if (zs->max_strm > 1) {
+ zs->max_strm--;
+ pr_err("%s: reduce max_comp_streams to %d\n",
+ "Low memory",
+ zs->max_strm);
+ }
wait_event(zs->strm_wait, !list_empty(&zs->idle_strm));


A 'broken English' comment can shed some light on the idea.
Hopefully, by the time we reduce ->max_strm there are N (>1)
streams already. In the worst case we go down to a single stream,
but that is something what would have happened anyway -- we don't
have memory for N streams. We don't rollback the max_stream value
(at least not in this version); we basically don't know when the
low memory condition goes away -- may be never.

> So I tried to modify this by pre-allocating and solved it, even if there
> was small memory using in advance.
> But because of this patch, if there is an unneeded memory using.
> I want to try new patch from another angle.
> Firstly, we can change 'vmalloc()' instead of 'kzalloc()' for the
> 'zstrm->private".

Yes, we can try this, I guess.

> Secondly, we can use GFP_NOWAIT flag to avoid this warning after 2nd
> stream allocation.

GFP_NOWAIT. hm... no, I don't think so.

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