Message-ID: <fb238383-a67b-47c1-94a7-0782fb0a4fe1@gmail.com>
Date: Thu, 5 Sep 2024 20:19:17 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Barry Song <21cnbao@...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 05/09/2024 12:00, Barry Song wrote:
> 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;
> 
LGTM with this swap_read_folio_zeromap. Thanks!

>         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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