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; 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
