On Thu, Nov 30, 2023 at 1:06 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: > > On 29/11/2023 07:47, Barry Song wrote: > >> Hi All, > >> > >> This is v3 of a series to add support for swapping out small-sized THP without > >> needing to first split the large folio via __split_huge_page(). It closely > >> follows the approach already used by PMD-sized THP. > >> > >> "Small-sized THP" is an upcoming feature that enables performance improvements > >> by allocating large folios for anonymous memory, where the large folio size is > >> smaller than the traditional PMD-size. See [3]. > >> > >> In some circumstances I've observed a performance regression (see patch 2 for > >> details), and this series is an attempt to fix the regression in advance of > >> merging small-sized THP support. > >> > >> I've done what I thought was the smallest change possible, and as a result, this > >> approach is only employed when the swap is backed by a non-rotating block device > >> (just as PMD-sized THP is supported today). Discussion against the RFC concluded > >> that this is probably sufficient. > >> > >> The series applies against mm-unstable (1a3c85fa684a) > >> > >> > >> Changes since v2 [2] > >> ==================== > >> > >> - Reuse scan_swap_map_try_ssd_cluster() between order-0 and order > 0 > >> allocation. This required some refactoring to make everything work nicely > >> (new patches 2 and 3). > >> - Fix bug where nr_swap_pages would say there are pages available but the > >> scanner would not be able to allocate them because they were reserved for the > >> per-cpu allocator. We now allow stealing of order-0 entries from the high > >> order per-cpu clusters (in addition to exisiting stealing from order-0 > >> per-cpu clusters). > >> > >> Thanks to Huang, Ying for the review feedback and suggestions! > >> > >> > >> Changes since v1 [1] > >> ==================== > >> > >> - patch 1: > >> - Use cluster_set_count() instead of cluster_set_count_flag() in > >> swap_alloc_cluster() since we no longer have any flag to set. I was unable > >> to kill cluster_set_count_flag() as proposed against v1 as other call > >> sites depend explicitly setting flags to 0. > >> - patch 2: > >> - Moved large_next[] array into percpu_cluster to make it per-cpu > >> (recommended by Huang, Ying). > >> - large_next[] array is dynamically allocated because PMD_ORDER is not > >> compile-time constant for powerpc (fixes build error). > >> > >> > >> Thanks, > >> Ryan > > > >> P.S. I know we agreed this is not a prerequisite for merging small-sized THP, > >> but given Huang Ying had provided some review feedback, I wanted to progress it. > >> All the actual prerequisites are either complete or being worked on by others. > >> > > > > Hi Ryan, > > > > this is quite important to a phone and a must-have component, so is large-folio > > swapin, as i explained to you in another email. > > Yes understood; the "prerequisites" are just the things that must be merged > *before* small-sized THP to ensure we don't regress existing behaviour or to > ensure that small-size THP is correct/robust when enabled. Performance > improvements can be merged after the initial small-sized series. I completely agree. I didn't mean small-THP swap out as a whole should be a prerequisite for small-THP initial patchset, just describing how important it is to a phone :-) And actually we have done much further than this on phones by optimizing zsmalloc/zram and allow a large folio compressed and decompressed as a whole, we have seen compressing/decompressing a whole large folio can significantly improve compression ratio and decrease CPU consumption. so that means large folios can not only save memory but also decrease CPU consumption. > > > Luckily, we are having Chuanhua Han(Cc-ed) to prepare a patchset of largefolio > > swapin on top of your this patchset, probably a port and cleanup of our > > do_swap_page[1] againest yours. > > That's great to hear - welcome aboard, Chuanhua Han! Feel free to reach out if > you have questions. > > I would guess that any large swap-in changes would be independent of this > swap-out patch though? Wouldn't you just be looking for contiguous swap entries > in the page table to determine a suitable folio order, then swap-in each of > those entries into the folio? And if they happen to have contiguous swap offsets > (enabled by this swap-out series) then you potentially get a batched disk access > benefit. I agree. Maybe we still need to check if the number of contiguous swap entries is one of those supported large folio sizes? > > That's just a guess though, perhaps you can describe your proposed approach? we have an ugly hack if we are swapping in from the dedicated zRAM for large folios, we assume we have a chance to swapin as a whole, but we do also handle corner cases in which some entries might have been zap_pte_range()-ed. My current proposal is as below, A1. we get the number of contiguous swap entries with PTL and find it is a valid large folio size A2. we allocate large folio without PTL A3. after getting PTL again, we re-check PTEs if the situation in A1 have been changed, if no other threads change those PTEs, we set_ptes and finish the swap-in but we have a chance to fail in A2, so in this case we still need to fall back to basepage. considering the MTE thread[1] I am handling, and MTE tag life cycle is the same with swap entry life cycle. it seems we will still need a page-level arch_swap_restore even after we support large folio swap-in for the below two reasons 1. contiguous PTEs might be partially dropped by madvise(DONTNEED) etc 2. we can still fall back to basepage for swap-in if we fail to get large folio even PTEs are all contiguous swap entries Of course, if we succeed in setting all PTEs for a large folio in A3, we can have a folio-level arch_swap_restore. To me, an universal folio-level arch_swap_restore seems not sensible to handle all kinds of complex cases. [1] [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios https://lore.kernel.org/linux-mm/20231114014313.67232-1-v-songbaohua@xxxxxxxx/ > > > > > Another concern is that swapslots can be fragmented, if we place small/large folios > > in a swap device, since large folios always require contiguous swapslot, we can > > result in failure of getting slots even we still have many free slots which are not > > contiguous. > > This series tries to mitigate that problem by reserving a swap cluster per > order. That works well until we run out of swap clusters; a cluster can't be > freed until all contained swap entries are swapped back in and deallocated. > > But I think we should start with the simple approach first, and only solve the > problems as they arise through real testing. I agree. > > To avoid this, [2] dynamic hugepage solution have two swap devices, > > one for basepage, the other one for CONTPTE. we have modified the priority-based > > selection of swap devices to choose swap devices based on small/large folios. > > i realize this approache is super ugly and might be very hard to find a way to > > upstream though, it seems not universal especially if you are a linux server (-_-) > > > > two devices are not a nice approach though it works well for a real product, > > we might still need some decent way to address this problem while the problem > > is for sure not a stopper of your patchset. > > I guess that approach works for your case because A) you only have 2 sizes, and > B) your swap device is zRAM, which dynamically allocate RAM as it needs it. > > The upstream small-sized THP solution can support multiple sizes, so you would > need a swap device per size (I think 13 is the limit at the moment - PMD size > for 64K base page). And if your swap device is a physical block device, you > can't dynamically parition it the way you can with zRAM. Nether of those things > scale particularly well IMHO. right. > > > > > [1] https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/mm/memory.c#L4648 > > [2] https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/mm/swapfile.c#L1129 > > > >> > >> [1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@xxxxxxx/ > >> [2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@xxxxxxx/ > >> [3] https://lore.kernel.org/linux-mm/15a52c3d-9584-449b-8228-1335e0753b04@xxxxxxx/ > >> > >> > >> Ryan Roberts (4): > >> mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags > >> mm: swap: Remove struct percpu_cluster > >> mm: swap: Simplify ssd behavior when scanner steals entry > >> mm: swap: Swap-out small-sized THP without splitting > >> > >> include/linux/swap.h | 31 +++--- > >> mm/huge_memory.c | 3 - > >> mm/swapfile.c | 232 ++++++++++++++++++++++++------------------- > >> mm/vmscan.c | 10 +- > >> 4 files changed, 149 insertions(+), 127 deletions(-) > > Thanks Barry
