On 05/09/2024 12:00, Barry Song wrote: > On Thu, Sep 5, 2024 at 10:53 PM Usama Arif <usamaarif642@xxxxxxxxx> wrote: >> >> >> >> On 05/09/2024 11:33, Barry Song wrote: >>> On Thu, Sep 5, 2024 at 10:10 PM Barry Song <21cnbao@xxxxxxxxx> 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); >>>> 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
