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Wed, 13 Nov 2019 10:08:22 +1100 (AEDT) Received: from hqpgpgate101.nvidia.com (Not Verified[216.228.121.13]) by hqemgate15.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA) id ; Tue, 12 Nov 2019 15:08:14 -0800 Received: from hqmail.nvidia.com ([172.20.161.6]) by hqpgpgate101.nvidia.com (PGP Universal service); Tue, 12 Nov 2019 15:08:15 -0800 X-PGP-Universal: processed; by hqpgpgate101.nvidia.com on Tue, 12 Nov 2019 15:08:15 -0800 Received: from [10.110.48.28] (172.20.13.39) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Tue, 12 Nov 2019 23:08:14 +0000 Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM To: Dan Williams References: <20191112000700.3455038-1-jhubbard@nvidia.com> <20191112000700.3455038-9-jhubbard@nvidia.com> <471e513c-833f-2f8b-60db-5d9c56a8f766@nvidia.com> From: John Hubbard X-Nvconfidentiality: public Message-ID: <729a16cb-3947-c7cb-c57f-6c917d240665@nvidia.com> Date: Tue, 12 Nov 2019 15:08:14 -0800 User-Agent: Mozilla/5.0 (X11; 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Miller" , Mike Kravetz Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 11/12/19 2:43 PM, Dan Williams wrote: ... > Ah, sorry. This was the first time I had looked at this series and > jumped in without reading the background. > > Your patch as is looks ok, I assume you've removed the FOLL_LONGTERM > warning in get_user_pages_remote in another patch? > Actually, I haven't gone quite that far. Actually this patch is the last change to that function. Therefore, at the end of this patchset, get_user_pages_remote() ends up with this check in it which is a less-restrictive version of the warning: /* * Current FOLL_LONGTERM behavior is incompatible with * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on * vmas. However, this only comes up if locked is set, and there are * callers that do request FOLL_LONGTERM, but do not set locked. So, * allow what we can. */ if (gup_flags & FOLL_LONGTERM) { if (WARN_ON_ONCE(locked)) return -EINVAL; } Is that OK, or did you want to go further (possibly in a follow-up patchset, as I'm hoping to get this one in soon)? ... >>> I think check_vma_flags() should do the ((FOLL_LONGTERM | FOLL_GET) && >>> vma_is_fsdax()) check and that would also remove the need for >>> __gup_longterm_locked. >>> >> >> Good idea, but there is still the call to check_and_migrate_cma_pages(), >> inside __gup_longterm_locked(). So it's a little more involved and >> we can't trivially delete __gup_longterm_locked() yet, right? > > [ add Aneesh ] > > Yes, you're right. I had overlooked that had snuck in there. That to > me similarly needs to be pushed down into the core with its own FOLL > flag, or it needs to be an explicit fixup that each caller does after > get_user_pages. The fact that migration silently happens as a side > effect of gup is too magical for my taste. > Yes. It's an intrusive side effect that is surprising, and not in a "happy surprise" way. :) . Fixing up the CMA pages by splitting that functionality into separate function calls sounds like an improvement worth exploring. thanks, -- John Hubbard NVIDIA