RE: [RFC] mm: remove swapcache page early

Dan Magenheimer
2013-03-27T22:24:38Z

> From: Hugh Dickins [mailto:hughd@google.com]
> Subject: Re: [RFC] mm: remove swapcache page early
>
> On Wed, 27 Mar 2013, Minchan Kim wrote:
>
> > Swap subsystem does lazy swap slot free with expecting the page
> > would be swapped out again so we can't avoid unnecessary write.
> so we can avoid unnecessary write.
> >
> > But the problem in in-memory swap is that it consumes memory space
> > until vm_swap_full(ie, used half of all of swap device) condition
> > meet. It could be bad if we use multiple swap device, small in-memory swap
> > and big storage swap or in-memory swap alone.
>
> That is a very good realization: it's surprising that none of us
> thought of it before - no disrespect to you, well done, thank you.

Yes, my compliments also Minchan. This problem has been thought of before
but this patch is the first to identify a possible solution.

> And I guess swap readahead is utterly unhelpful in this case too.

Yes... as is any "swap writeahead". Excuse my ignorance, but I
think this is not done in the swap subsystem but instead the kernel
assumes write-coalescing will be done in the block I/O subsystem,
which means swap writeahead would affect zram but not zcache/zswap
(since frontswap subverts the block I/O subsystem).

However I think a swap-readahead solution would be helpful to
zram as well as zcache/zswap.

> > This patch changes vm_swap_full logic slightly so it could free
> > swap slot early if the backed device is really fast.
> > For it, I used SWP_SOLIDSTATE but It might be controversial.
>
> But I strongly disagree with almost everything in your patch :)
> I disagree with addressing it in vm_swap_full(), I disagree that
> it can be addressed by device, I disagree that it has anything to
> do with SWP_SOLIDSTATE.
>
> This is not a problem with swapping to /dev/ram0 or to /dev/zram0,
> is it? In those cases, a fixed amount of memory has been set aside
> for swap, and it works out just like with disk block devices. The
> memory set aside may be wasted, but that is accepted upfront.

It is (I believe) also a problem with swapping to ram. Two
copies of the same page are kept in memory in different places,
right? Fixed vs variable size is irrelevant I think. Or am
I misunderstanding something about swap-to-ram?

> Similarly, this is not a problem with swapping to SSD. There might
> or might not be other reasons for adjusting the vm_swap_full() logic
> for SSD or generally, but those have nothing to do with this issue.

I think it is at least highly related. The key issue is the
tradeoff of the likelihood that the page will soon be read/written
again while it is in swap cache vs the time/resource-usage necessary
to "reconstitute" the page into swap cache. Reconstituting from disk
requires a LOT of elapsed time. Reconstituting from
an SSD likely takes much less time. Reconstituting from
zcache/zram takes thousands of CPU cycles.

> The problem here is peculiar to frontswap, and the variably sized
> memory behind it, isn't it? We are accustomed to using swap to free
> up memory by transferring its data to some other, cheaper but slower
> resource.

Frontswap does make the problem more complex because some pages
are in "fairly fast" storage (zcache, needs decompression) and
some are on the actual (usually) rotating media. Fortunately,
differentiating between these two cases is just a table lookup
(see frontswap_test).

> But in the case of frontswap and zmem (I'll say that to avoid thinking
> through which backends are actually involved), it is not a cheaper and
> slower resource, but the very same memory we are trying to save: swap
> is stolen from the memory under reclaim, so any duplication becomes
> counter-productive (if we ignore cpu compression/decompression costs:
> I have no idea how fair it is to do so, but anyone who chooses zmem
> is prepared to pay some cpu price for that).

Exactly. There is some "robbing of Peter to pay Paul" and
other complex resource tradeoffs. Presumably, though, it is
not "the very same memory we are trying to save" but a
fraction of it, saving the same page of data more efficiently
in memory, using less than a page, at some CPU cost.

> And because it's a frontswap thing, we cannot decide this by device:
> frontswap may or may not stand in front of each device. There is no
> problem with swapcache duplicated on disk (until that area approaches
> being full or fragmented), but at the higher level we cannot see what
> is in zmem and what is on disk: we only want to free up the zmem dup.

I *think* frontswap_test(page) resolves this problem, as long as
we have a specific page available to use as a parameter.

> I believe the answer is for frontswap/zmem to invalidate the frontswap
> copy of the page (to free up the compressed memory when possible) and
> SetPageDirty on the PageUptodate PageSwapCache page when swapping in
> (setting page dirty so nothing will later go to read it from the
> unfreed location on backing swap disk, which was never written).

There are two duplication issues: (1) When can the page be removed
from the swap cache after a call to frontswap_store; and (2) When
can the page be removed from the frontswap storage after it
has been brought back into memory via frontswap_load.

This patch from Minchan addresses (1). The issue you are raising
here is (2). You may not know that (2) has recently been solved
in frontswap, at least for zcache. See frontswap_exclusive_gets_enabled.
If this is enabled (and it is for zcache but not yet for zswap),
what you suggest (SetPageDirty) is what happens.

> We cannot rely on freeing the swap itself, because in general there
> may be multiple references to the swap, and we only satisfy the one
> which has faulted. It may or may not be a good idea to use rmap to
> locate the other places to insert pte in place of swap entry, to
> resolve them all at once; but we have chosen not to do so in the
> past, and there's no need for that, if the zmem gets invalidated
> and the swapcache page set dirty.

I see. Minchan's patch handles the removal "reactively"... it
might be possible to handle it more proactively. Or it may
be possible to take the number of references into account when
deciding whether to frontswap_store the page as, presumably,
the likelihood of needing to "reconstitute" the page sooner increases
with each additional reference.

> Hugh

Very useful thoughts, Hugh. Thanks much and looking forward
to more discussion at LSF/MM!

Dan

P.S. When I refer to zcache, I am referring to the version in
drivers/staging/zcache in 3.9. The code in drivers/staging/zcache
in 3.8 is "old zcache"... "new zcache" is in drivers/staging/ramster
in 3.8. Sorry for any confusion...