Message-ID: <CAGsJ_4yVB4wcqp5cDpnj6XSuqtb4G=DRiMBssbhpTcVSwaQd6A@mail.gmail.com>
Date: Thu, 5 Sep 2024 22:42:03 +1200
From: Barry Song <21cnbao@...il.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Yosry Ahmed <yosryahmed@...gle.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 10:37 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 05/09/2024 11:10, Barry Song wrote:
> > On Thu, Sep 5, 2024 at 8:49 PM Barry Song <21cnbao@...il.com> wrote:
> >>
> >> 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.
> >
> > No, returning 0 after only checking first entry would still reintroduce
> > the current bug, where the start entry is zeromap but other entries
> > might not be. We need another value to indicate whether the entries
> > are consistent if we want to avoid the enum:
> >
> > /*
> >  * Return the number of contiguous zeromap entries started from entry;
> >  * If all entries have consistent zeromap, *consistent will be true;
> >  * otherwise, false;
> >  */
> > static inline unsigned int swap_zeromap_entries_count(swp_entry_t entry,
> >                 int nr, bool *consistent)
> > {
> >         struct swap_info_struct *sis = swp_swap_info(entry);
> >         unsigned long start = swp_offset(entry);
> >         unsigned long end = start + nr;
> >         unsigned long s_idx, c_idx;
> >
> >         s_idx = find_next_bit(sis->zeromap, end, start);
>
> In all of the implementations you sent, you are using find_next_bit(..,end, start), but
> I believe it should be find_next_bit(..,nr, start)?

I guess no, the tricky thing is that size means the size from the first bit of
bitmap but not from the "start" bit?

>
> TBH, I liked the enum implementation you had in https://lore.kernel.org/all/20240905002926.1055-1-21cnbao@gmail.com/
> Its the easiest to review and understand, and least likely to introduce any bugs.
> But it could be a personal preference.
> The likelihood of having contiguous zeromap entries *that* is less than nr is very low right?
> If so we could go with the enum implementation?

what about the bool impementation i sent in the last email, it seems the
simplest code.

>
>
> >         if (s_idx == end) {
> >                 *consistent = true;
> >                 return 0;
> >         }
> >
> >         c_idx = find_next_zero_bit(sis->zeromap, end, start);
> >         if (c_idx == end) {
> >                 *consistent = true;
> >                 return nr;
> >         }
> >
> >         *consistent = false;
> >         if (s_idx == start)
> >                 return 0;
> >         return c_idx - s_idx;
> > }
> >
> > I can actually switch the places of the "consistent" and returned
> > number if that looks
> > better.
> >
> >>
> >> 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

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