From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A8EDFC3A5A8 for ; Wed, 4 Sep 2019 17:01:15 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 57DD021670 for ; Wed, 4 Sep 2019 17:01:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 57DD021670 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=codewreck.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id D89276B0003; Wed, 4 Sep 2019 13:01:14 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D394F6B0006; Wed, 4 Sep 2019 13:01:14 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C76BA6B0007; Wed, 4 Sep 2019 13:01:14 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0116.hostedemail.com [216.40.44.116]) by kanga.kvack.org (Postfix) with ESMTP id A8E0B6B0003 for ; Wed, 4 Sep 2019 13:01:14 -0400 (EDT) Received: from smtpin02.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with SMTP id 58CB0180AD804 for ; Wed, 4 Sep 2019 17:01:14 +0000 (UTC) X-FDA: 75897853668.02.roof93_44e27ddd97931 X-HE-Tag: roof93_44e27ddd97931 X-Filterd-Recvd-Size: 4560 Received: from nautica.notk.org (nautica.notk.org [91.121.71.147]) by imf07.hostedemail.com (Postfix) with ESMTP for ; Wed, 4 Sep 2019 17:01:13 +0000 (UTC) Received: by nautica.notk.org (Postfix, from userid 1001) id 94F13C009; Wed, 4 Sep 2019 19:01:11 +0200 (CEST) Date: Wed, 4 Sep 2019 19:00:56 +0200 From: Dominique Martinet To: Matthew Wilcox Cc: linux-mm@kvack.org Subject: Re: How to use huge pages in drivers? Message-ID: <20190904170056.GA9825@nautica> References: <20190903182627.GA6079@nautica> <20190903184230.GJ29434@bombadil.infradead.org> <20190903212815.GA7518@nautica> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190903212815.GA7518@nautica> User-Agent: Mutt/1.5.21 (2010-09-15) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Dominique Martinet wrote on Tue, Sep 03, 2019: > Matthew Wilcox wrote on Tue, Sep 03, 2019: > > > What I'd like to know is: > > > - we know (assuming the other side isn't too bugged, but if it is we're > > > fucked up anyway) exactly what huge-page-sized physical memory range has > > > been mapped on the other side, is there a way to manually gather the > > > pages corresponding and merge them into a huge page? > > > > You're using the word 'page' here, but I suspect what you really mean is > > "pfn" or "pte". As you've described it, it doesn't matter what data structure > > Linux is using for the memory, since Linux doesn't know about the memory. > > Correct, we're already using vmf_insert_pfn Actually let me take that back, vmf_insert_pfn is only used if pfn_valid() is false, probably as a safeguard of sort(?). The normal case went with pfn_to_page(pfn) + vm_insert_page() so, as things stands. I do have a few more questions if you could humor me a bit more... - the vma was created with a vm_flags including VM_MIXEDMAP for some reason, I don't know why. If I change it to VM_PFNMAP (which sounds better here from the little I understand of this as we do not need cow and looks a bit simpler?), I can remove the vm_insert_page() path and use the vmf_insert_pfn one instead, which appears to work fine for simple programs... But the kernel thread for my network adapter (bxi... which is not upstream either I guess.. sigh..) no longer tries to fault via my custom .fault vm operation... Which means I probably did need MIXEDMAP ? I'm honestly not sure where to read up on what these two flags imply, looking at the page fault handler code I do not see why the request from a kernel thread would care what kind of vma it is... - ignoring that for now (it's not like I need to switch to PFNMAP); adding vmf_insert_pfn_pmd() for when the remote side uses large pages, it complains that the vmf->pmd is not a pmd_none nor huge nor a devmap (this check appears specific to rhel7 kernel, I could temporarily test with an upstream kernel but the network adapter won't work there so I'll need this to work on this ultimately) It looks like handle_mm_fault() will always try to allocate a pmd so it should never be empty in my fault handler, and I don't see anything else than vmf_insert_pfn_pmd() setting the mkdevmap flag, and it's not huge either... (on a dump, the the pmd content is 175cb18067, so these flags according to crash for x86_64 are (PRESENT|RW|USER|ACCESSED|DIRTY)) I tried adding a huge_fault vm op thinking it might be called with a more appropriate pmd but it doesn't seem to be called at all in my case..? I would have assumed from the code that it would try every page and if I try to somehow force it by using pmd_mkdevmap on the vmf->pmd, things appear to work until the process exits and zap_page does a null deref on pgtable_trans_huge_withdraw because the pgtable was never deposited - this looks gone on newer kernels, but once again I do not see where these should come from; I'm just assuming I reap what I sew messing with the flags. Long story short, I think I have some deeper undestanding problem about the whole thing. Do I also need to use some specific flags when that special file is mmap'd to allow huge_fault to be called ? I think transparent_hugepage_enabled(vma) is fine, but the vmf.pmd found in __handle_mm_fault is probably already not none at this point...? Thanks again, feel free to ignore me for a bit longer I'll keep digging my own grave, writing to a rubber duck that might have an idea of how far the wrong way I've gone already helps... :D -- Dominique