Re: [RFC 3/3] zram: add swap_get_free hint

From: Minchan Kim
Date: Fri Sep 19 2014 - 02:14:28 EST

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In reply to: Dan Streetman: "Re: [RFC 3/3] zram: add swap_get_free hint"
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On Wed, Sep 17, 2014 at 12:28:59PM -0400, Dan Streetman wrote:
> On Wed, Sep 17, 2014 at 3:44 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > On Tue, Sep 16, 2014 at 11:58:32AM -0400, Dan Streetman wrote:
> >> On Mon, Sep 15, 2014 at 9:21 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >> > On Mon, Sep 15, 2014 at 12:00:33PM -0400, Dan Streetman wrote:
> >> >> On Sun, Sep 14, 2014 at 8:57 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >> >> > On Sat, Sep 13, 2014 at 03:39:13PM -0400, Dan Streetman wrote:
> >> >> >> On Thu, Sep 4, 2014 at 7:59 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >> >> >> > Hi Heesub,
> >> >> >> >
> >> >> >> > On Thu, Sep 04, 2014 at 03:26:14PM +0900, Heesub Shin wrote:
> >> >> >> >> Hello Minchan,
> >> >> >> >>
> >> >> >> >> First of all, I agree with the overall purpose of your patch set.
> >> >> >> >
> >> >> >> > Thank you.
> >> >> >> >
> >> >> >> >>
> >> >> >> >> On 09/04/2014 10:39 AM, Minchan Kim wrote:
> >> >> >> >> >This patch implement SWAP_GET_FREE handler in zram so that VM can
> >> >> >> >> >know how many zram has freeable space.
> >> >> >> >> >VM can use it to stop anonymous reclaiming once zram is full.
> >> >> >> >> >
> >> >> >> >> >Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> >> >> >> >> >---
> >> >> >> >> > drivers/block/zram/zram_drv.c | 18 ++++++++++++++++++
> >> >> >> >> > 1 file changed, 18 insertions(+)
> >> >> >> >> >
> >> >> >> >> >diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> >> >> >> >> >index 88661d62e46a..8e22b20aa2db 100644
> >> >> >> >> >--- a/drivers/block/zram/zram_drv.c
> >> >> >> >> >+++ b/drivers/block/zram/zram_drv.c
> >> >> >> >> >@@ -951,6 +951,22 @@ static int zram_slot_free_notify(struct block_device *bdev,
> >> >> >> >> > return 0;
> >> >> >> >> > }
> >> >> >> >> >
> >> >> >> >> >+static int zram_get_free_pages(struct block_device *bdev, long *free)
> >> >> >> >> >+{
> >> >> >> >> >+ struct zram *zram;
> >> >> >> >> >+ struct zram_meta *meta;
> >> >> >> >> >+
> >> >> >> >> >+ zram = bdev->bd_disk->private_data;
> >> >> >> >> >+ meta = zram->meta;
> >> >> >> >> >+
> >> >> >> >> >+ if (!zram->limit_pages)
> >> >> >> >> >+ return 1;
> >> >> >> >> >+
> >> >> >> >> >+ *free = zram->limit_pages - zs_get_total_pages(meta->mem_pool);
> >> >> >> >>
> >> >> >> >> Even if 'free' is zero here, there may be free spaces available to
> >> >> >> >> store more compressed pages into the zs_pool. I mean calculation
> >> >> >> >> above is not quite accurate and wastes memory, but have no better
> >> >> >> >> idea for now.
> >> >> >> >
> >> >> >> > Yeb, good point.
> >> >> >> >
> >> >> >> > Actually, I thought about that but in this patchset, I wanted to
> >> >> >> > go with conservative approach which is a safe guard to prevent
> >> >> >> > system hang which is terrible than early OOM kill.
> >> >> >> >
> >> >> >> > Whole point of this patchset is to add a facility to VM and VM
> >> >> >> > collaborates with zram via the interface to avoid worst case
> >> >> >> > (ie, system hang) and logic to throttle could be enhanced by
> >> >> >> > several approaches in future but I agree my logic was too simple
> >> >> >> > and conservative.
> >> >> >> >
> >> >> >> > We could improve it with [anti|de]fragmentation in future but
> >> >> >> > at the moment, below simple heuristic is not too bad for first
> >> >> >> > step. :)
> >> >> >> >
> >> >> >> >
> >> >> >> > ---
> >> >> >> > drivers/block/zram/zram_drv.c | 15 ++++++++++-----
> >> >> >> > drivers/block/zram/zram_drv.h | 1 +
> >> >> >> > 2 files changed, 11 insertions(+), 5 deletions(-)
> >> >> >> >
> >> >> >> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> >> >> >> > index 8e22b20aa2db..af9dfe6a7d2b 100644
> >> >> >> > --- a/drivers/block/zram/zram_drv.c
> >> >> >> > +++ b/drivers/block/zram/zram_drv.c
> >> >> >> > @@ -410,6 +410,7 @@ static bool zram_free_page(struct zram *zram, size_t index)
> >> >> >> > atomic64_sub(zram_get_obj_size(meta, index),
> >> >> >> > &zram->stats.compr_data_size);
> >> >> >> > atomic64_dec(&zram->stats.pages_stored);
> >> >> >> > + atomic_set(&zram->alloc_fail, 0);
> >> >> >> >
> >> >> >> > meta->table[index].handle = 0;
> >> >> >> > zram_set_obj_size(meta, index, 0);
> >> >> >> > @@ -600,10 +601,12 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> >> >> >> > alloced_pages = zs_get_total_pages(meta->mem_pool);
> >> >> >> > if (zram->limit_pages && alloced_pages > zram->limit_pages) {
> >> >> >> > zs_free(meta->mem_pool, handle);
> >> >> >> > + atomic_inc(&zram->alloc_fail);
> >> >> >> > ret = -ENOMEM;
> >> >> >> > goto out;
> >> >> >> > }
> >> >> >>
> >> >> >> This isn't going to work well at all with swap. There will be,
> >> >> >> minimum, 32 failures to write a swap page before GET_FREE finally
> >> >> >> indicates it's full, and even then a single free during those 32
> >> >> >> failures will restart the counter, so it could be dozens or hundreds
> >> >> >> (or more) swap write failures before the zram device is marked as
> >> >> >> full. And then, a single zram free will move it back to non-full and
> >> >> >> start the write failures over again.
> >> >> >>
> >> >> >> I think it would be better to just check for actual fullness (i.e.
> >> >> >> alloced_pages > limit_pages) at the start of write, and fail if so.
> >> >> >> That will allow a single write to succeed when it crosses into
> >> >> >> fullness, and the if GET_FREE is changed to a simple IS_FULL and uses
> >> >> >> the same check (alloced_pages > limit_pages), then swap shouldn't see
> >> >> >> any write failures (or very few), and zram will stay full until enough
> >> >> >> pages are freed that it really does move under limit_pages.
> >> >> >
> >> >> > The alloced_pages > limit_pages doesn't mean zram is full so with your
> >> >> > approach, it could kick OOM earlier which is not what we want.
> >> >> > Because our product uses zram to delay app killing by low memory killer.
> >> >>
> >> >> With zram, the meaning of "full" isn't as obvious as other fixed-size
> >> >> storage devices. Obviously, "full" usually means "no more room to
> >> >> store anything", while "not full" means "there is room to store
> >> >> anything, up to the remaining free size". With zram, its zsmalloc
> >> >> pool size might be over the specified limit, but there will still be
> >> >> room to store *some* things - but not *anything*. Only compressed
> >> >> pages that happen to fit inside a class with at least one zspage that
> >> >> isn't full.
> >> >>
> >> >> Clearly, we shouldn't wait to declare zram "full" only once zsmalloc
> >> >> is 100% full in all its classes.
> >> >>
> >> >> What about waiting until there is N number of write failures, like
> >> >> this patch? That doesn't seem very fair to the writer, since each
> >> >> write failure will cause them to do extra work (first, in selecting
> >> >> what to write, and then in recovering from the failed write).
> >> >> However, it will probably squeeze some writes into some of those empty
> >> >> spaces in already-allocated zspages.
> >> >>
> >> >> And declaring zram "full" immediately once the zsmalloc pool size
> >> >> increases past the specified limit? Since zsmalloc's classes almost
> >> >> certainly contain some fragmentation, that will waste all the empty
> >> >> spaces that could still store more compressed pages. But, this is the
> >> >> limit at which you cannot guarantee all writes to be able to store a
> >> >> compressed page - any zsmalloc classes without a partially empty
> >> >> zspage will have to increase zsmalloc's size, therefore failing the
> >> >> write.
> >> >>
> >> >> Neither definition of "full" is optimal. Since in this case we're
> >> >> talking about swap, I think forcing swap write failures to happen,
> >> >> which with direct reclaim could (I believe) stop everything while the
> >> >> write failures continue, should be avoided as much as possible. Even
> >> >> when zram fullness is delayed by N write failures, to try to squeeze
> >> >> out as much storage from zsmalloc as possible, when it does eventually
> >> >> fill if zram is the only swap device the system will OOM anyway. And
> >> >> if zram isn't the only swap device, but just the first (highest
> >> >> priority), then delaying things with unneeded write failures is
> >> >> certainly not better than just filling up so swap can move on to the
> >> >> next swap device. The only case where write failures delaying marking
> >> >> zram as full will help is if the system stopped right at this point,
> >> >> and then started decreasing how much memory was needed. That seems
> >> >> like a very unlikely coincidence, but maybe some testing would help
> >> >> determine how bad the write failures affect system
> >> >> performance/responsiveness and how long they delay OOM.
> >> >
> >> > Please, keep in mind that swap is alreay really slow operation but
> >> > we want to use it to avoid OOM if possible so I can't buy your early
> >> > kill suggestion.
> >>
> >> I disagree, OOM should be invoked once the system can't proceed with
> >> reclaiming memory. IMHO, repeated swap write failures will cause the
> >> system to be unable to reclaim memory.
> >
> > That's what I want. I'd like to go with OOM once repeated swap write
> > failures happens.
> > The difference between you and me is that how we should be aggressive
> > to kick OOM. Your proposal was too agressive so that it can make OOM
> > too early, which makes swap inefficient. That's what I'd like to avoid.
> >
> >>
> >> > If a user feel it's really slow for his product,
> >> > it means his admin was fail. He should increase the limit of zram
> >> > dynamically or statically(zram already support that ways).
> >> >
> >> > The thing I'd like to solve in this patchset is to avoid system hang
> >> > where admin cannot do anyting, even ctrl+c, which is thing should
> >> > support in OS level.
> >>
> >> what's better - failing a lot of swap writes, or marking the swap
> >> device as full? As I said if zram is the *only* swap device in the
> >> system, maybe that makes sense (although it's still questionable). If
> >> zram is only the first swap device, and there's a backup swap device
> >> (presumably that just writes to disk), then it will be *much* better
> >> to simply fail over to that, instead of (repeatedly) failing a lot of
> >> swap writes.
> >
> > Actually, I don't hear such usecase until now but I can't ignore it
> > because it's really doable configuration so I agree we need some knob.
> >
> >>
> >> Especially once direct reclaim is reached, failing swap writes is
> >> probably going to make the system unresponsive. Personally I think
> >> moving to OOM (or the next swap device) is better.
> >
> > Again said, that's what I want! But your suggestion was too agressive.
> > The system can have more resource which can free easily(ex, page cache,
> > purgeable memory or unimportant process could be killed).
>
> Ok, I think we agree - I'm not against some write failures, I just
> worry about "too many" (where I can't define "too many" ;-) of them,
> since each write failure doesn't make any progress in reclaiming
> memory for the process(es) that are waiting for it.
>
> Also when you got the write errors, I assume you saw a lot of:
> Write-error on swap-device (%u:%u:%Lu)
> messages? Obviously that's expected, but maybe it would be good to
> add a check for swap_hint IS_FULL there, and skip printing the alert
> if so...?

If it's really problem, we could add ratelimit like buffer_io_error.

>
> >
> >>
> >> If write failures are the direction you go, then IMHO there should *at
> >> least* be a zram parameter to allow the admin to choose to immediately
> >> fail or continue with write failures.
> >
> > Agree.
> >
> >>
> >>
> >> >
> >> >>
> >> >> Since there may be different use cases that desire different things,
> >> >> maybe there should be a zram runtime (or buildtime) config to choose
> >> >> exactly how it decides it's full? Either full after N write failures,
> >> >> or full when alloced>limit? That would allow the user to either defer
> >> >> getting full as long as possible (at the possible cost of system
> >> >> unresponsiveness during those write failures), or to just move
> >> >> immediately to zram being full as soon as it can't guarantee that each
> >> >> write will succeed.
> >> >
> >> > Hmm, I thought it and was going to post it when I send v1.
> >> > My idea was this.
> >>
> >> what i actually meant was more like this, where ->stop_using_when_full
> >> is a user-configurable param:
> >>
> >> bool zram_is_full(...)
> >> {
> >> if (zram->stop_using_when_full) {
> >> /* for this, allow 1 write to succeed past limit_pages */
> >> return zs_get_total_pages(zram) > zram->limit_pages;
> >> } else {
> >> return zram->alloc_fail > ALLOC_FAIL_THRESHOLD;
> >> }
> >> }
> >
> > To me, it's too simple so there is no chance to control zram fullness.
> > How about this one?
> >
> > bool zram_is_full(...)
> > {
> > unsigned long total_pages;
> > if (!zram->limit_pages)
> > return false;
> >
> > total_pages = zs_get_total_pages(zram);
> > if (total_pages >= zram->limit_pages &&
>
> Just to clarify - this also implies that zram_bvec_write() will allow
> (at least) one write to succeed past limit_pages (which I agree
> with)...

Yeb.

>
> > (100 * (compr_data_size >> PAGE_SHIFT) / total_pages) > FRAG_THRESH_HOLD)
>
> I assume FRAG_THRESH_HOLD will be a runtime user-configurable param?

Sure.

>
> Also strictly as far as semantics, it seems like this is the reverse
> of fragmentation, i.e. high fragmentation should indicate lots of
> empty space, while low fragmentation should indicate compr_data_size
> is almost equal to total_pages. Not a big deal, but may cause
> confusion. Maybe call it "efficiency" or "compaction"? Or invert the
> calculation, e.g.
> (100 * (total_pages - compr_pages) / total_pages) < FRAG_THRESH_HOLD
>
> > return true;
> >
> > if (zram->alloc_fail > FULL_THRESH_HOLD)
> > return true;
> >
> > return false;
> > }

I'd like to define "fullness".

fullness = (100 * used space / total space)

if (100 * compr_pages / total_pages) >= ZRAM_FULLNESS_PERCENT)
return 1;

It means the higher fullness is, the slower we reach zram full.

And I want to set ZRAM_FULLNESS_PERCENT as 80, which means want
to bias more memory consumption rather than early OOM kill.

> >
> > So if someone want to avoid write failure but bear with early OOM,
> > he can set FRAG_THRESH_HOLD to 0.
> > Any thought?
>
> Yep that looks good.
>
> One minor english correction, "threshold" is a single word.

Thanks for the review, Dan!

>
> >
> >>
> >> >
> >> > int zram_get_free_pages(...)
> >> > {
> >> > if (zram->limit_pages &&
> >> > zram->alloc_fail > FULL_THRESH_HOLD &&
> >> > (100 * compr_data_size >> PAGE_SHIFT /
> >> > zs_get_total_pages(zram)) > FRAG_THRESH_HOLD) {
> >>
> >> well...i think this implementation has both downsides; it forces write
> >> failures to happen, but also it doesn't guarantee being full after
> >> FULL_THRESHOLD write failures. If the fragmentation level never
> >> reaches FRAG_THRESHOLD, it'll fail writes forever. I can't think of
> >> any way that using the amount of fragmentation will work, because you
> >> can't guarantee it will be reached. The incoming pages to compress
> >> may all fall into classes that are already full.
> >>
> >> with zsmalloc compaction, it would be possible to know that a certain
> >> fragmentation threshold could be reached, but without it that's not a
> >> promise zsmalloc can keep. And we definitely don't want to fail swap
> >> writes forever.
> >>
> >>
> >> > *free = 0;
> >> > return 0;
> >> > }
> >> > ..
> >> > }
> >> >
> >> > Maybe we could export FRAG_THRESHOLD.
> >> >
> >> >>
> >> >>
> >> >>
> >> >> >
> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >> >
> >> >> >> > + atomic_set(&zram->alloc_fail, 0);
> >> >> >> > update_used_max(zram, alloced_pages);
> >> >> >> >
> >> >> >> > cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_WO);
> >> >> >> > @@ -951,6 +954,7 @@ static int zram_slot_free_notify(struct block_device *bdev,
> >> >> >> > return 0;
> >> >> >> > }
> >> >> >> >
> >> >> >> > +#define FULL_THRESH_HOLD 32
> >> >> >> > static int zram_get_free_pages(struct block_device *bdev, long *free)
> >> >> >> > {
> >> >> >> > struct zram *zram;
> >> >> >> > @@ -959,12 +963,13 @@ static int zram_get_free_pages(struct block_device *bdev, long *free)
> >> >> >> > zram = bdev->bd_disk->private_data;
> >> >> >> > meta = zram->meta;
> >> >> >> >
> >> >> >> > - if (!zram->limit_pages)
> >> >> >> > - return 1;
> >> >> >> > -
> >> >> >> > - *free = zram->limit_pages - zs_get_total_pages(meta->mem_pool);
> >> >> >> > + if (zram->limit_pages &&
> >> >> >> > + (atomic_read(&zram->alloc_fail) > FULL_THRESH_HOLD)) {
> >> >> >> > + *free = 0;
> >> >> >> > + return 0;
> >> >> >> > + }
> >> >> >> >
> >> >> >> > - return 0;
> >> >> >> > + return 1;
> >> >> >>
> >> >> >> There's no way that zram can even provide a accurate number of free
> >> >> >> pages, since it can't know how compressible future stored pages will
> >> >> >> be. It would be better to simply change this swap_hint from GET_FREE
> >> >> >> to IS_FULL, and return either true or false.
> >> >> >
> >> >> > My plan is that we can give an approximation based on
> >> >> > orig_data_size/compr_data_size with tweaking zero page and vmscan can use
> >> >> > the hint from get_nr_swap_pages to throttle file/anon balance but I want to do
> >> >> > step by step so I didn't include the hint.
> >> >> > If you are strong against with that in this stage, I can change it and
> >> >> > try it later with the number.
> >> >> > Please, say again if you want.
> >> >>
> >> >> since as you said zram is the only user of swap_hint, changing it
> >> >> later shouldn't be a big deal. And you could have both, IS_FULL and
> >> >> GET_FREE; since the check in scan_swap_map() really only is checking
> >> >> for IS_FULL, if you update vmscan later to adjust its file/anon
> >> >> balance based on GET_FREE, that can be added then with no trouble,
> >> >> right?
> >> >
> >> > Yeb, No problem.
> >> >
> >> >>
> >> >>
> >> >> >
> >> >> > Thanks for the review!
> >> >> >
> >> >> >
> >> >> >>
> >> >> >>
> >> >> >> > }
> >> >> >> >
> >> >> >> > static int zram_swap_hint(struct block_device *bdev,
> >> >> >> > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> >> >> >> > index 779d03fa4360..182a2544751b 100644
> >> >> >> > --- a/drivers/block/zram/zram_drv.h
> >> >> >> > +++ b/drivers/block/zram/zram_drv.h
> >> >> >> > @@ -115,6 +115,7 @@ struct zram {
> >> >> >> > u64 disksize; /* bytes */
> >> >> >> > int max_comp_streams;
> >> >> >> > struct zram_stats stats;
> >> >> >> > + atomic_t alloc_fail;
> >> >> >> > /*
> >> >> >> > * the number of pages zram can consume for storing compressed data
> >> >> >> > */
> >> >> >> > --
> >> >> >> > 2.0.0
> >> >> >> >
> >> >> >> >>
> >> >> >> >> heesub
> >> >> >> >>
> >> >> >> >> >+
> >> >> >> >> >+ return 0;
> >> >> >> >> >+}
> >> >> >> >> >+
> >> >> >> >> > static int zram_swap_hint(struct block_device *bdev,
> >> >> >> >> > unsigned int hint, void *arg)
> >> >> >> >> > {
> >> >> >> >> >@@ -958,6 +974,8 @@ static int zram_swap_hint(struct block_device *bdev,
> >> >> >> >> >
> >> >> >> >> > if (hint == SWAP_SLOT_FREE)
> >> >> >> >> > ret = zram_slot_free_notify(bdev, (unsigned long)arg);
> >> >> >> >> >+ else if (hint == SWAP_GET_FREE)
> >> >> >> >> >+ ret = zram_get_free_pages(bdev, arg);
> >> >> >> >> >
> >> >> >> >> > return ret;
> >> >> >> >> > }
> >> >> >> >> >
> >> >> >> >>
> >> >> >> >> --
> >> >> >> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> >> >> >> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> >> >> >> >> see: http://www.linux-mm.org/ .
> >> >> >> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >> >> >> >
> >> >> >> > --
> >> >> >> > Kind regards,
> >> >> >> > Minchan Kim
> >> >> >>
> >> >> >> --
> >> >> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> >> >> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> >> >> >> see: http://www.linux-mm.org/ .
> >> >> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >> >> >
> >> >> > --
> >> >> > Kind regards,
> >> >> > Minchan Kim
> >> >>
> >> >> --
> >> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> >> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> >> >> see: http://www.linux-mm.org/ .
> >> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >> >
> >> > --
> >> > Kind regards,
> >> > Minchan Kim
> >>
> >> --
> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >
> > --
> > Kind regards,
> > Minchan Kim
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

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Minchan Kim
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In reply to: Dan Streetman: "Re: [RFC 3/3] zram: add swap_get_free hint"
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