Message-ID: <CAGsJ_4xRHtFvxc6kbpGdrvxpaQYCHNWpZsMO+GbX7LJwW841nw@mail.gmail.com>
Date: Thu, 5 Sep 2024 23:00:39 +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 10:53 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 05/09/2024 11:33, Barry Song wrote:
> > On Thu, Sep 5, 2024 at 10:10 PM Barry Song <21cnbao@...il.com> 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);
> >>         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.
> >
> > I'd rather make it simpler by:
> >
> > /*
> >  * Check if all entries have consistent zeromap status, return true if
> >  * all entries are zeromap or non-zeromap, else return false;
> >  */
> > static inline bool 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;
> >
> I guess you meant end= start + nr here?

right.

>
> >         if (find_next_bit(sis->zeromap, end, start) == end)
> >                 return true;
> >         if (find_next_zero_bit(sis->zeromap, end, start) == end)
> >                 return true;
> >
> So if zeromap is all false, this still returns true. We cant use this function in swap_read_folio_zeromap,
> to check at time of swapin if all were zeros, right?

We can, my point is that swap_read_folio_zeromap() is the only
function that actually
needs the real value of zeromap; the others only care about
consistency. So if we can
avoid introducing a new enum across modules, we avoid it :-)

static bool swap_read_folio_zeromap(struct folio *folio)
{
        struct swap_info_struct *sis = swp_swap_info(folio->swap)
        unsigned int nr_pages = folio_nr_pages(folio);
        swp_entry_t entry = folio->swap;

        /*
         * Swapping in a large folio that is partially in the zeromap is not
         * currently handled. Return true without marking the folio uptodate so
         * that an IO error is emitted (e.g. do_swap_page() will sigbus).
         */
        if (WARN_ON_ONCE(!swap_zeromap_entries_check(entry, nr_pages)))
                return true;

        if (!test_bit(swp_offset(entry), sis->zeromap))
                return false;

        folio_zero_range(folio, 0, folio_size(folio));
        folio_mark_uptodate(folio);
        return true;
}

mm/memory.c only needs true or false, it doesn't care about the real value.

>
>
> >         return false;
> > }
> >
> > mm/page_io.c can combine this with reading the zeromap of first entry to
> > decide if it will read folio from zeromap; mm/memory.c only needs the bool
> > to fallback to the largest possible order.
> >
> > static inline unsigned long thp_swap_suitable_orders(...)
> > {
> >         int order, nr;
> >
> >         order = highest_order(orders);
> >
> >         while (orders) {
> >                 nr = 1 << order;
> >                 if ((addr >> PAGE_SHIFT) % nr == swp_offset % nr &&
> >                     swap_zeromap_entries_check(entry, nr))
> >                         break;
> >                 order = next_order(&orders, order);
> >         }
> >
> >         return orders;
> > }
> >
> >>
> >>>
> >>> 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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