On 05/09/2024 11:10, Barry Song wrote: > On Thu, Sep 5, 2024 at 8:49 PM Barry Song <21cnbao@xxxxxxxxx> wrote: >> >> On Thu, Sep 5, 2024 at 7:55 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: >>> >>> On Thu, Sep 5, 2024 at 12:03 AM Barry Song <21cnbao@xxxxxxxxx> wrote: >>>> >>>> On Thu, Sep 5, 2024 at 5:41 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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@xxxxxxxxx/ >>>> >>>> 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)? TBH, I liked the enum implementation you had in https://lore.kernel.org/all/20240905002926.1055-1-21cnbao@xxxxxxxxx/ 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? > 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
