Ralph Campbell <rcampbell@xxxxxxxxxx> writes: > On 6/22/20 4:54 PM, Yang Shi wrote: >> On Mon, Jun 22, 2020 at 4:02 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: >>> >>> On 2020-06-22 15:33, Yang Shi wrote: >>>> On Mon, Jun 22, 2020 at 3:30 PM Yang Shi <shy828301@xxxxxxxxx> wrote: >>>>> On Mon, Jun 22, 2020 at 2:53 PM Zi Yan <ziy@xxxxxxxxxx> wrote: >>>>>> On 22 Jun 2020, at 17:31, Ralph Campbell wrote: >>>>>>> On 6/22/20 1:10 PM, Zi Yan wrote: >>>>>>>> On 22 Jun 2020, at 15:36, Ralph Campbell wrote: >>>>>>>>> On 6/21/20 4:20 PM, Zi Yan wrote: >>>>>>>>>> On 19 Jun 2020, at 17:56, Ralph Campbell wrote: >>> ... >>>>>> Ying(cc’d) developed the code to swapout and swapin THP in one piece: https://lore.kernel.org/linux-mm/20181207054122.27822-1-ying.huang@xxxxxxxxx/. >>>>>> I am not sure whether the patchset makes into mainstream or not. It could be a good technical reference >>>>>> for swapping in device private pages, although swapping in pages from disk and from device private >>>>>> memory are two different scenarios. >>>>>> >>>>>> Since the device private memory swapin impacts core mm performance, we might want to discuss your patches >>>>>> with more people, like the ones from Ying’s patchset, in the next version. >>>>> >>>>> I believe Ying will give you more insights about how THP swap works. >>>>> >>>>> But, IMHO device memory migration (migrate to system memory) seems >>>>> like THP CoW more than swap. >>> >>> >>> A fine point: overall, the desired behavior is "migrate", not CoW. >>> That's important. Migrate means that you don't leave a page behind, even >>> a read-only one. And that's exactly how device private migration is >>> specified. >>> >>> We should try to avoid any erosion of clarity here. Even if somehow >>> (really?) the underlying implementation calls this THP CoW, the actual >>> goal is to migrate pages over to the device (and back). >>> >>> >>>>> >>>>> When migrating in: >>>> >>>> Sorry for my fat finger, hit sent button inadvertently, let me finish here. >>>> >>>> When migrating in: >>>> >>>> - if THP is enabled: allocate THP, but need handle allocation >>>> failure by falling back to base page >>>> - if THP is disabled: fallback to base page >>>> >>> >>> OK, but *all* page entries (base and huge/large pages) need to be cleared, >>> when migrating to device memory, unless I'm really confused here. >>> So: not CoW. >> >> I realized the comment caused more confusion. I apologize for the >> confusion. Yes, the trigger condition for swap/migration and CoW are >> definitely different. Here I mean the fault handling part of migrating >> into system memory. >> >> Swap-in just needs to handle the base page case since THP swapin is >> not supported in upstream yet and the PMD is split in swap-out phase >> (see shrink_page_list). >> >> The patch adds THP migration support to device memory, but you need to >> handle migrate in (back to system memory) case correctly. The fault >> handling should look like THP CoW fault handling behavior (before >> 5.8): >> - if THP is enabled: allocate THP, fallback if allocation is failed >> - if THP is disabled: fallback to base page >> >> Swap fault handling doesn't look like the above. So, I said it seems >> like more THP CoW (fault handling part only before 5.8). I hope I >> articulate my mind. >> >> However, I didn't see such fallback is handled. It looks if THP >> allocation is failed, it just returns SIGBUS; and no check about THP >> status if I read the patches correctly. The THP might be disabled for >> the specific vma or system wide before migrating from device memory >> back to system memory. > > You are correct, the patch wasn't handling the fallback case. > I'll add that in the next version. For fallback, you need to split the THP in device firstly. Because you will migrate a base page instead a whole THP now. Best Regards, Huang, Ying >>> >>> thanks, >>> -- >>> John Hubbard >>> NVIDIA
