On 8/23/21 9:21 PM, Dan Williams wrote: > On Mon, Aug 23, 2021 at 12:47 PM Gerald Schaefer > <gerald.schaefer@xxxxxxxxxxxxx> wrote: >> >> On Mon, 23 Aug 2021 16:05:46 +0200 >> Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> wrote: >> >>> On Fri, 20 Aug 2021 07:43:40 +0200 >>> Christoph Hellwig <hch@xxxxxx> wrote: >>> >>>> Hi all, >>>> >>>> looking at the recent ZONE_DEVICE related changes we still have a >>>> horrible maze of different code paths. I already suggested to >>>> depend on ARCH_HAS_PTE_SPECIAL for ZONE_DEVICE there, which all modern >> >> Oh, we do have PTE_SPECIAL, actually that took away the last free bit >> in the pte. So, if there is a chance that ZONE_DEVICE would depend >> on PTE_SPECIAL instead of PTE_DEVMAP, we might be back in the game >> and get rid of that CONFIG_FS_DAX_LIMITED. > > So PTE_DEVMAP is primarily there to coordinate the > get_user_pages_fast() path, and even there it's usage can be > eliminated in favor of PTE_SPECIAL. I started that effort [1], but > need to rebase on new notify_failure infrastructure coming from Ruan > [2]. So I think you are not in the critical path until I can get the > PTE_DEVMAP requirement out of your way. > Isn't the implicit case that PTE_SPECIAL means that you aren't supposed to get a struct page back? The gup path bails out on pte_special() case. And in the fact in this thread that you quote: > [1]: https://lore.kernel.org/r/161604050866.1463742.7759521510383551055.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (...) we were speaking about[1.1] using that same special bit to block longterm gup for fs-dax (while allowing it device-dax which does support it). [1.1] https://lore.kernel.org/nvdimm/a8c41028-c7f5-9b93-4721-b8ddcf2427da@xxxxxxxxxx/ Or maybe that's what you mean for this particular case of FS_DAX_LIMITED. Most _special*() cases in mm match _devmap*() as far I've experimented in the past with PMD/PUD and dax (prior to [1.1]). I am just wondering would you differentiate the case where you have metadata for the !FS_DAX_LIMITED case in {gup,gup_fast} path in light of removing PTE_DEVMAP. I would have thought of checking that a pgmap exists for the pfn (without grabbing a ref to it). > > [2]: https://lore.kernel.org/r/20210730085245.3069812-1-ruansy.fnst@xxxxxxxxxxx >
