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[70.31.26.132]) by smtp.gmail.com with ESMTPSA id q30-20020a37f71e000000b006faaf6dc55asm11148662qkj.22.2022.12.21.11.23.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Dec 2022 11:23:38 -0800 (PST) Date: Wed, 21 Dec 2022 14:23:35 -0500 From: Peter Xu To: James Houghton Cc: Mike Kravetz , Muchun Song , David Hildenbrand , David Rientjes , Axel Rasmussen , Mina Almasry , Zach O'Keefe , Manish Mishra , Naoya Horiguchi , "Dr . David Alan Gilbert" , "Matthew Wilcox (Oracle)" , Vlastimil Babka , Baolin Wang , Miaohe Lin , Yang Shi , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 33/47] userfaultfd: add UFFD_FEATURE_MINOR_HUGETLBFS_HGM Message-ID: References: <20221021163703.3218176-1-jthoughton@google.com> <20221021163703.3218176-34-jthoughton@google.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Queue-Id: E4D0D40019 X-Stat-Signature: 3w8prmmmmt8dkomycjuq8omfxye9nf9f X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1671650621-38550 X-HE-Meta: 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 Bu/Y13dT bwP5N3juS0ompHsKsxQ+EI2iiOlKRQX+M5WzfPooEdGxYGrSwXxaGZsHtHUb/HUx6EgLxJH7bZL3n868lQoVOIRbA1IBXurw4+6fH9LPnj7m2SODOrTEYFBpBavec9pyi3QWKIYtsA3tCaJPV13WwUr8kXQ== 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: James, On Wed, Nov 16, 2022 at 03:30:00PM -0800, James Houghton wrote: > On Wed, Nov 16, 2022 at 2:28 PM Peter Xu wrote: > > > > On Fri, Oct 21, 2022 at 04:36:49PM +0000, James Houghton wrote: > > > Userspace must provide this new feature when it calls UFFDIO_API to > > > enable HGM. Userspace can check if the feature exists in > > > uffdio_api.features, and if it does not exist, the kernel does not > > > support and therefore did not enable HGM. > > > > > > Signed-off-by: James Houghton > > > > It's still slightly a pity that this can only be enabled by an uffd context > > plus a minor fault, so generic hugetlb users cannot directly leverage this. > > The idea here is that, for applications that can conceivably benefit > from HGM, we have a mechanism for enabling it for that application. So > this patch creates that mechanism for userfaultfd/UFFDIO_CONTINUE. I > prefer this approach over something more general like MADV_ENABLE_HGM > or something. Sorry to get back to this very late - I know this has been discussed since the very early stage of the feature, but is there any reasoning behind? When I start to think seriously on applying this to process snapshot with uffd-wp I found that the minor mode trick won't easily play - normally that's a case where all the pages were there mapped huge, but when the app wants UFFDIO_WRITEPROTECT it may want to remap the huge pages into smaller pages, probably some size that the user can specify. It'll be non-trivial to enable HGM during that phase using MINOR mode because in that case the pages are all mapped. For the long term, I am just still worried the current interface is still not as flexible. -- Peter Xu