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; if (find_next_bit(sis->zeromap, end, start) == end) return true; if (find_next_zero_bit(sis->zeromap, end, start) == end) return true; 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
