Message-ID: <CAGsJ_4x0y+RtghmFifm_pR-=P_t5hNW5qjvw-oF+-T_amuVuzQ@mail.gmail.com>
Date: Thu, 5 Sep 2024 20:49:31 +1200
From: Barry Song <21cnbao@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: usamaarif642@...il.com, akpm@...ux-foundation.org, 
	chengming.zhou@...ux.dev, david@...hat.com, hannes@...xchg.org, 
	hughd@...gle.com, kernel-team@...a.com, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, nphamcs@...il.com, shakeel.butt@...ux.dev, 
	willy@...radead.org, ying.huang@...el.com, hanchuanhua@...o.com
Subject: Re: [PATCH v4 1/2] mm: store zero pages to be swapped out in a bitmap

On Thu, Sep 5, 2024 at 7:55 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Thu, Sep 5, 2024 at 12:03 AM Barry Song <21cnbao@...il.com> wrote:
> >
> > On Thu, Sep 5, 2024 at 5:41 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > >
> > > [..]
> > > > > I understand the point of doing this to unblock the synchronous large
> > > > > folio swapin support work, but at some point we're gonna have to
> > > > > actually handle the cases where a large folio being swapped in is
> > > > > partially in the swap cache, zswap, the zeromap, etc.
> > > > >
> > > > > All these cases will need similar-ish handling, and I suspect we won't
> > > > > just skip swapping in large folios in all these cases.
> > > >
> > > > I agree that this is definitely the goal. `swap_read_folio()` should be a
> > > > dependable API that always returns reliable data, regardless of whether
> > > > `zeromap` or `zswap` is involved. Despite these issues, mTHP swap-in shouldn't
> > > > be held back. Significant efforts are underway to support large folios in
> > > > `zswap`, and progress is being made. Not to mention we've already allowed
> > > > `zeromap` to proceed, even though it doesn't support large folios.
> > > >
> > > > It's genuinely unfair to let the lack of mTHP support in `zeromap` and
> > > > `zswap` hold swap-in hostage.
> > >
> >
> > Hi Yosry,
> >
> > > Well, two points here:
> > >
> > > 1. I did not say that we should block the synchronous mTHP swapin work
> > > for this :) I said the next item on the TODO list for mTHP swapin
> > > support should be handling these cases.
> >
> > Thanks for your clarification!
> >
> > >
> > > 2. I think two things are getting conflated here. Zswap needs to
> > > support mTHP swapin*. Zeromap already supports mTHPs AFAICT. What is
> > > truly, and is outside the scope of zswap/zeromap, is being able to
> > > support hybrid mTHP swapin.
> > >
> > > When swapping in an mTHP, the swapped entries can be on disk, in the
> > > swapcache, in zswap, or in the zeromap. Even if all these things
> > > support mTHPs individually, we essentially need support to form an
> > > mTHP from swap entries in different backends. That's what I meant.
> > > Actually if we have that, we may not really need mTHP swapin support
> > > in zswap, because we can just form the large folio in the swap layer
> > > from multiple zswap entries.
> > >
> >
> > After further consideration, I've actually started to disagree with the idea
> > of supporting hybrid swapin (forming an mTHP from swap entries in different
> > backends). My reasoning is as follows:
>
> I do not have any data about this, so you could very well be right
> here. Handling hybrid swapin could be simply falling back to the
> smallest order we can swapin from a single backend. We can at least
> start with this, and collect data about how many mTHP swapins fallback
> due to hybrid backends. This way we only take the complexity if
> needed.
>
> I did imagine though that it's possible for two virtually contiguous
> folios to be swapped out to contiguous swap entries and end up in
> different media (e.g. if only one of them is zero-filled). I am not
> sure how rare it would be in practice.
>
> >
> > 1. The scenario where an mTHP is partially zeromap, partially zswap, etc.,
> > would be an extremely rare case, as long as we're swapping out the mTHP as
> > a whole and all the modules are handling it accordingly. It's highly
> > unlikely to form this mix of zeromap, zswap, and swapcache unless the
> > contiguous VMA virtual address happens to get some small folios with
> > aligned and contiguous swap slots. Even then, they would need to be
> > partially zeromap and partially non-zeromap, zswap, etc.
>
> As I mentioned, we can start simple and collect data for this. If it's
> rare and we don't need to handle it, that's good.
>
> >
> > As you mentioned, zeromap handles mTHP as a whole during swapping
> > out, marking all subpages of the entire mTHP as zeromap rather than just
> > a subset of them.
> >
> > And swap-in can also entirely map a swapcache which is a large folio based
> > on our previous patchset which has been in mainline:
> > "mm: swap: entirely map large folios found in swapcache"
> > https://lore.kernel.org/all/20240529082824.150954-1-21cnbao@gmail.com/
> >
> > It seems the only thing we're missing is zswap support for mTHP.
>
> It is still possible for two virtually contiguous folios to be swapped
> out to contiguous swap entries. It is also possible that a large folio
> is swapped out as a whole, then only a part of it is swapped in later
> due to memory pressure. If that part is later reclaimed again and gets
> added to the swapcache, we can run into the hybrid swapin situation.
> There may be other scenarios as well, I did not think this through.
>
> >
> > 2. Implementing hybrid swap-in would be extremely tricky and could disrupt
> > several software layers. I can share some pseudo code below:
>
> Yeah it definitely would be complex, so we need proper justification for it.
>
> >
> > swap_read_folio()
> > {
> >        if (zeromap_full)
> >                folio_read_from_zeromap()
> >        else if (zswap_map_full)
> >               folio_read_from_zswap()
> >        else {
> >               folio_read_from_swapfile()
> >               if (zeromap_partial)
> >                        folio_read_from_zeromap_fixup()  /* fill zero
> > for partially zeromap subpages */
> >               if (zwap_partial)
> >                        folio_read_from_zswap_fixup()  /* zswap_load
> > for partially zswap-mapped subpages */
> >
> >                folio_mark_uptodate()
> >                folio_unlock()
> > }
> >
> > We'd also need to modify folio_read_from_swapfile() to skip
> > folio_mark_uptodate()
> > and folio_unlock() after completing the BIO. This approach seems to
> > entirely disrupt
> > the software layers.
> >
> > This could also lead to unnecessary IO operations for subpages that
> > require fixup.
> > Since such cases are quite rare, I believe the added complexity isn't worth it.
> >
> > My point is that we should simply check that all PTEs have consistent zeromap,
> > zswap, and swapcache statuses before proceeding, otherwise fall back to the next
> > lower order if needed. This approach improves performance and avoids complex
> > corner cases.
>
> Agree that we should start with that, although we should probably
> fallback to the largest order we can swapin from a single backend,
> rather than the next lower order.
>
> >
> > So once zswap mTHP is there, I would also expect an API similar to
> > swap_zeromap_entries_check()
> > for example:
> > zswap_entries_check(entry, nr) which can return if we are having
> > full, non, and partial zswap to replace the existing
> > zswap_never_enabled().
>
> I think a better API would be similar to what Usama had. Basically
> take in (entry, nr) and return how much of it is in zswap starting at
> entry, so that we can decide the swapin order.
>
> Maybe we can adjust your proposed swap_zeromap_entries_check() as well
> to do that? Basically return the number of swap entries in the zeromap
> starting at 'entry'. If 'entry' itself is not in the zeromap we return
> 0 naturally. That would be a small adjustment/fix over what Usama had,
> but implementing it with bitmap operations like you did would be
> better.

I assume you means the below

/*
 * Return the number of contiguous zeromap entries started from entry
 */
static inline unsigned int swap_zeromap_entries_count(swp_entry_t entry, int nr)
{
        struct swap_info_struct *sis = swp_swap_info(entry);
        unsigned long start = swp_offset(entry);
        unsigned long end = start + nr;
        unsigned long idx;

        idx = find_next_bit(sis->zeromap, end, start);
        if (idx != start)
                return 0;

        return find_next_zero_bit(sis->zeromap, end, start) - idx;
}

If yes, I really like this idea.

It seems much better than using an enum, which would require adding a new
data structure :-) Additionally, returning the number allows callers
to fall back
to the largest possible order, rather than trying next lower orders
sequentially.

Hi Usama,
what is your take on this?

>
> >
> > Though I am not sure how cheap zswap can implement it,
> > swap_zeromap_entries_check()
> > could be two simple bit operations:
> >
> > +static inline zeromap_stat_t swap_zeromap_entries_check(swp_entry_t
> > entry, int nr)
> > +{
> > +       struct swap_info_struct *sis = swp_swap_info(entry);
> > +       unsigned long start = swp_offset(entry);
> > +       unsigned long end = start + nr;
> > +
> > +       if (find_next_bit(sis->zeromap, end, start) == end)
> > +               return SWAP_ZEROMAP_NON;
> > +       if (find_next_zero_bit(sis->zeromap, end, start) == end)
> > +               return SWAP_ZEROMAP_FULL;
> > +
> > +       return SWAP_ZEROMAP_PARTIAL;
> > +}
> >
> > 3. swapcache is different from zeromap and zswap. Swapcache indicates
> > that the memory
> > is still available and should be re-mapped rather than allocating a
> > new folio. Our previous
> > patchset has implemented a full re-map of an mTHP in do_swap_page() as mentioned
> > in 1.
> >
> > For the same reason as point 1, partial swapcache is a rare edge case.
> > Not re-mapping it
> > and instead allocating a new folio would add significant complexity.
> >
> > > >
> > > > Nonetheless, `zeromap` and `zswap` are distinct cases. With `zeromap`, we
> > > > permit almost all mTHP swap-ins, except for those rare situations where
> > > > small folios that were swapped out happen to have contiguous and aligned
> > > > swap slots.
> > > >
> > > > swapcache is another quite different story, since our user scenarios begin from
> > > > the simplest sync io on mobile phones, we don't quite care about swapcache.
> > >
> > > Right. The reason I bring this up is as I mentioned above, there is a
> > > common problem of forming large folios from different sources, which
> > > includes the swap cache. The fact that synchronous swapin does not use
> > > the swapcache was a happy coincidence for you, as you can add support
> > > mTHP swapins without handling this case yet ;)
> >
> > As I mentioned above, I'd really rather filter out those corner cases
> > than support
> > them, not just for the current situation to unlock swap-in series :-)
>
> If they are indeed corner cases, then I definitely agree.

Thanks
Barry

Powered by blists - more mailing lists