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. > 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. That's just a guess though, perhaps you can describe your proposed approach? > > 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. 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. > > [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
