Re: [PATCH v5 3/4] zram: zram memory size limitation

From: Dan Streetman
Date: Wed Aug 27 2014 - 12:29:50 EST

On Wed, Aug 27, 2014 at 11:35 AM, David Horner <ds2horner@xxxxxxxxx> wrote:
> On Wed, Aug 27, 2014 at 11:14 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
>> On Wed, Aug 27, 2014 at 10:44 AM, David Horner <ds2horner@xxxxxxxxx> wrote:
>>> On Wed, Aug 27, 2014 at 10:03 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
>>>> On Tue, Aug 26, 2014 at 10:51 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>>>>> Hey Joonsoo,
>>>>> On Wed, Aug 27, 2014 at 10:26:11AM +0900, Joonsoo Kim wrote:
>>>>>> Hello, Minchan and David.
>>>>>> On Tue, Aug 26, 2014 at 08:22:29AM -0400, David Horner wrote:
>>>>>> > On Tue, Aug 26, 2014 at 3:55 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>>>>>> > > Hey Joonsoo,
>>>>>> > >
>>>>>> > > On Tue, Aug 26, 2014 at 04:37:30PM +0900, Joonsoo Kim wrote:
>>>>>> > >> On Mon, Aug 25, 2014 at 09:05:55AM +0900, Minchan Kim wrote:
>>>>>> > >> > @@ -513,6 +540,14 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>>>>>> > >> > ret = -ENOMEM;
>>>>>> > >> > goto out;
>>>>>> > >> > }
>>>>>> > >> > +
>>>>>> > >> > + if (zram->limit_pages &&
>>>>>> > >> > + zs_get_total_pages(meta->mem_pool) > zram->limit_pages) {
>>>>>> > >> > + zs_free(meta->mem_pool, handle);
>>>>>> > >> > + ret = -ENOMEM;
>>>>>> > >> > + goto out;
>>>>>> > >> > + }
>>>>>> > >> > +
>>>>>> > >> > cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_WO);
>>>>>> > >>
>>>>>> > >> Hello,
>>>>>> > >>
>>>>>> > >> I don't follow up previous discussion, so I could be wrong.
>>>>>> > >> Why this enforcement should be here?
>>>>>> > >>
>>>>>> > >> I think that this has two problems.
>>>>>> > >> 1) alloc/free happens unnecessarilly if we have used memory over the
>>>>>> > >> limitation.
>>>>>> > >
>>>>>> > > True but firstly, I implemented the logic in zsmalloc, not zram but
>>>>>> > > as I described in cover-letter, it's not a requirement of zsmalloc
>>>>>> > > but zram so it should be in there. If every user want it in future,
>>>>>> > > then we could move the function into zsmalloc. That's what we
>>>>>> > > concluded in previous discussion.
>>>>>> Hmm...
>>>>>> Problem is that we can't avoid these unnecessary overhead in this
>>>>>> implementation. If we can implement this feature in zram efficiently,
>>>>>> it's okay. But, I think that current form isn't.
>>>>> If we can add it in zsmalloc, it would be more clean and efficient
>>>>> for zram but as I said, at the moment, I didn't want to put zram's
>>>>> requirement into zsmalloc because to me, it's weird to enforce max
>>>>> limit to allocator. It's client's role, I think.
>>>>> If current implementation is expensive and rather hard to follow,
>>>>> It would be one reason to move the feature into zsmalloc but
>>>>> I don't think it makes critical trobule in zram usecase.
>>>>> See below.
>>>>> But I still open and will wait others's opinion.
>>>>> If other guys think zsmalloc is better place, I am willing to move
>>>>> it into zsmalloc.
>>>> Moving it into zsmalloc would allow rejecting new zsmallocs before
>>>> actually crossing the limit, since it can calculate that internally.
>>>> However, with the current patches the limit will only be briefly
>>>> crossed, and it should not be crossed by a large amount. Now, if this
>>>> is happening repeatedly and quickly during extreme memory pressure,
>>>> the constant alloc/free will clearly be worse than a simple internal
>>>> calculation and failure. But would it ever happen repeatedly once the
>>>> zram limit is reached?
>>>> Now that I'm thinking about the limit from the perspective of the zram
>>>> user, I wonder what really will happen. If zram is being used for
>>>> swap space, then when swap starts getting errors trying to write
>>>> pages, how damaging will that be to the system? I haven't checked
>>>> what swap does when it encounters disk errors. Of course, with no
>>>> zram limit, continually writing to zram until memory is totally
>>>> consumed isn't good either. But in any case, I would hope that swap
>>>> would not repeatedly hammer on a disk when it's getting write failures
>>>> from it.
>>>> Alternately, if zram was being used as a compressed ram disk for
>>>> regular file storage, it's entirely up to the application to handle
>>>> write failures, so it may continue to try to write to a full zram
>>>> disk.
>>>> As far as what the zsmalloc api would look like with the limit added,
>>>> it would need a setter and getter function (adding it as a param to
>>>> the create function would be optional i think). But more importantly,
>>>> it would need to handle multiple ways of specifying the limit. In our
>>>> specific current use cases, zram and zswap, each handles their
>>>> internal limit differently - zswap currently uses a % of total ram as
>>>> its limit (defaulting to 20), while with these patches zram will use a
>>>> specific number of bytes as its limit (defaulting to no limit). If
>>>> the limiting mechanism is moved into zsmalloc (and possibly zbud),
>>>> then either both users need to use the same units (bytes or %ram), or
>>>> zsmalloc/zbud need to be able to set their limit in either units. It
>>>> seems to me like keeping the limit in zram/zswap is currently
>>>> preferable, at least without both using the same limit units.
>>> zswap knows what 20% (or whatever % it currently uses , and perhaps it too
>>> will become a tuning knob) of memory is in bytes.
>>> So, if the interface to establish a limit for a pool (or pool set, or whatever
>>> zsmalloc sets up for its allocation mechanism) is stipulated in bytes
>>> (to actually use pages internally, of visa-versa) , then both can use
>>> that interface.
>>> zram with its native page stipulation, and zswap with calculated % of memory).
>> No, unless zswap monitors memory hotplug and updates the limit on each
>> hotplug event, 20% of the *current* total ram at zswap initialization
>> is not equal to an actual 20% of ram limit. zswap checks its size
>> against totalram_pages for each new allocation. I don't think we would
>> prefer adding memory hotplug monitoring to zswap just to update the
>> zpool size limit.
> OK - I see the need to retain the limits where they are in the using
> components so that
> zsmalloc is not unnecessarily complicated (keeping track of 2 limit methods).
> So, zswap has the same race conditions and possible transient over-allocations?
> It looks like I will have to check on how zswap implements it.
> But perhaps you can answer the question that is not in the code:
> Have there been reported thrashing behaviour around the 20% limit for zswap?

zswap does a simple over-allocation check before allocating anything.
So during page store, it checks if (total_ram * 0.20) < used. This
actually places the effective limit higher than the specified limit,
but only by a single allocation. This approach could be taken with
zram as well.

The amount of over-allocation (past the specified limit) would vary
between zsmalloc and zbud. Since zbud increases itself in page
increments, any over-allocation past the zswap limit would be by only
1 page. However, zsmalloc is variable in its allocation increments,
as it depends on which class needs to be grown; zsmalloc is divided
into many "classes", each of contains some number of "zspages" which
try to precisely contain some number of N-sized areas; e.g. one class
might use zspages that are 2 pages to store 3 separate areas which are
each 2/3 of a page number of bytes; if that class needed to be grown,
it would add one zspage that is 2 pages. The max number of actual
pages per zspage is defined by ZS_MAX_PAGES_PER_ZSPAGE which is
currently set to 1<<2, so 4.

So with zswap, it will over-allocate memory past its specified limit,
up to 1 page (with zbud) or up to 4 pages (with zsmalloc). zram could
do the same, simply check if its size > limit before each write, and
fail if so; that would remove the alloc/free issue, and would only
over-allocate by at most 4 pages (with the current zsmalloc settings).
Alternately, zram could check if its (current_size + 4pages > limit),
which would then stop it short of the limit by up to 4 pages. Really
though, 4 pages either above or under the limit probably doesn't

> thanks.
>>> Both would need a mechanism to change the max as need change,
>>> so the API has to handle this.
>>> Or am I way off base?
>>>>>> > >
>>>>>> > > Another idea is we could call zs_get_total_pages right before zs_malloc
>>>>>> > > but the problem is we cannot know how many of pages are allocated
>>>>>> > > by zsmalloc in advance.
>>>>>> > > IOW, zram should be blind on zsmalloc's internal.
>>>>>> > >
>>>>>> >
>>>>>> > We did however suggest that we could check before hand to see if
>>>>>> > max was already exceeded as an optimization.
>>>>>> > (possibly with a guess on usage but at least using the minimum of 1 page)
>>>>>> > In the contested case, the max may already be exceeded transiently and
>>>>>> > therefore we know this one _could_ fail (it could also pass, but odds
>>>>>> > aren't good).
>>>>>> > As Minchan mentions this was discussed before - but not into great detail.
>>>>>> > Testing should be done to determine possible benefit. And as he also
>>>>>> > mentions, the better place for it may be in zsmalloc, but that
>>>>>> > requires an ABI change.
>>>>>> Why we hesitate to change zsmalloc API? It is in-kernel API and there
>>>>>> are just two users now, zswap and zram. We can change it easily.
>>>>>> I think that we just need following simple API change in zsmalloc.c.
>>>>>> zs_zpool_create(gfp_t gfp, struct zpool_ops *zpool_op)
>>>>>> =>
>>>>>> zs_zpool_create(unsigned long limit, gfp_t gfp, struct zpool_ops
>>>>>> *zpool_op)
>>>>>> It's pool allocator so there is no obstacle for us to limit maximum
>>>>>> memory usage in zsmalloc. It's a natural idea to limit memory usage
>>>>>> for pool allocator.
>>>>>> > Certainly a detailed suggestion could happen on this thread and I'm
>>>>>> > also interested
>>>>>> > in your thoughts, but this patchset should be able to go in as is.
>>>>>> > Memory exhaustion avoidance probably trumps the possible thrashing at
>>>>>> > threshold.
>>>>>> >
>>>>>> > > About alloc/free cost once if it is over the limit,
>>>>>> > > I don't think it's important to consider.
>>>>>> > > Do you have any scenario in your mind to consider alloc/free cost
>>>>>> > > when the limit is over?
>>>>>> > >
>>>>>> > >> 2) Even if this request doesn't do new allocation, it could be failed
>>>>>> > >> due to other's allocation. There is time gap between allocation and
>>>>>> > >> free, so legimate user who want to use preallocated zsmalloc memory
>>>>>> > >> could also see this condition true and then he will be failed.
>>>>>> > >
>>>>>> > > Yeb, we already discussed that. :)
>>>>>> > > Such false positive shouldn't be a severe problem if we can keep a
>>>>>> > > promise that zram user cannot exceed mem_limit.
>>>>>> > >
>>>>>> If we can keep such a promise, why we need to limit memory usage?
>>>>>> I guess that this limit feature is useful for user who can't keep such promise.
>>>>>> So, we should assume that this false positive happens frequently.
>>>>> The goal is to limit memory usage within some threshold.
>>>>> so false positive shouldn't be harmful unless it exceeds the threshold.
>>>>> In addition, If such false positive happens frequently, it means
>>>>> zram is very trobule so that user would see lots of write fail
>>>>> message, sometime really slow system if zram is used for swap.
>>>>> If we protect just one write from the race, how much does it help
>>>>> this situation? I don't think it's critical problem.
>>>>>> > And we cannot avoid the race, nor can we avoid in a low overhead competitive
>>>>>> > concurrent process transient inconsistent states.
>>>>>> > Different views for different observers.
>>>>>> > They are a consequence of the theory of "Special Computational Relativity".
>>>>>> > I am working on a String Unification Theory of Quantum and General CR in LISP.
>>>>>> > ;-)
>>>>>> If we move limit logic to zsmalloc, we can avoid the race by commiting
>>>>>> needed memory size before actual allocation attempt. This commiting makes
>>>>>> concurrent process serialized so there is no race here. There is
>>>>>> possibilty to fail to allocate, but I think this is better than alloc
>>>>>> and free blindlessly depending on inconsistent states.
>>>>> Normally, zsmalloc/zsfree allocates object from existing pool so
>>>>> it's not big overhead and if someone continue to try writing once limit is
>>>>> full, another overhead (vfs, fs, block) would be bigger than zsmalloc
>>>>> so it's not a problem, I think.
>>>>>> Thanks.
>>>>>> --
>>>>>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>>>>>> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
>>>>>> see: .
>>>>>> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
>>>>> --
>>>>> Kind regards,
>>>>> Minchan Kim
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