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Peter Anvin" , nh-open-source@amazon.com, Peter Xu , Sebastian Biemueller , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Matthew Wilcox , Andrew Morton , "linux-mm@kvack.org" Content-Type: text/plain; charset="us-ascii" X-Rspam-User: X-Rspamd-Queue-Id: E1476180012 X-Rspamd-Server: rspam01 X-Stat-Signature: kqqt8gy9xzbr5nrqwfsy1kz3pn7afc1f X-HE-Tag: 1723854152-986632 X-HE-Meta: U2FsdGVkX1/PmON9QQ8tLoUlQVOGcQCH19BCYfI+TqL2/+HgAUmKnRdeB24XcMT+AgaPCjPqp7qapcYU8ig0KorWUM+YCaYwBsC5vR2CMQ6cBzgec5Lrh5WNlSQ8gWq1DUyzL94kZdPXOCDCBZ3qY4Jb6j4UK0TVg+EUJRLUD+IAUSakGSxdW+W1ymCQqNfSyBL0BAAiRO4hjd9bpgTE7nyREFi4PT/U0+fv9l4gOjTtj6liSeMcLYseb+fINWBPf6Ff5bD91Kp3E40B8hyixAOqrVhvq5HFe1+Ki8aSds6mNbSvBzajkztj/IFBJuGKSaQL5DlpbbQj/qKMGObWSaHE79UijFNd4FbcO2lASZGkhL0qlJ4Kf803HRLrh/fRSS1inLOvNS8NUF4KpJTtLP9AUVudKW3PV+vBegodoXbU6wdvXi4aX2vpPYQOjQnTI1Z+pmA7IAnSQWvMDLF+W4bRzP2I+8igKaC6L8dReRil0Xti7Tn+1VR0OUCFINK05Rau735b0HBZqaUKH/Jro0m7eJ+uE+hxGH7EuJofaefCdnv9EqrosuZfWUtalLwXo2iLB9OHqV3BqWeIo4OJUdLnTVtk1TaPGIemceAAu6QsMyotYYN11/hbE43wbIE31fkIRXogRNQ9gdjDRSqFlBX9zbbUaoas0ftK33TxIMUEcvbZBfnfWKpc3Na3znCq3uzhXo/HJ4/6YVvV/cCUBObuiRk9NWa7FQoBTin/ZaV1yX8WeR8d4yw/IhmAym20bli0D7yuxyLvvLWcNmJXSlyAmZLLCFtShacCIuZ6ljiVlW16o0g1B1qGWEfpDQLGljo8MzVu59bgKC2nYXq//t6yJCyidLjMZcOqjH4WmJNb/tqJtS+dXYE6UNxYmNdrSMVNUaNWpK56ZFiQB7VUOqFn0jusdSnMH3MpOsevFjnWBIJu46peasrkuBH5lQrGdvzH2VA9PBz3Zb0PlUK 2ggeV+Cr 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 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: List-Subscribe: List-Unsubscribe: On Fri, Aug 02, 2024, David Woodhouse wrote: > On Fri, 2024-08-02 at 11:44 +0000, Carsten Stollmaier wrote: > > On vcpu_run, before entering the guest, the update of the steal time > > information causes a page-fault if the page is not present. In our > > scenario, this gets handled by do_user_addr_fault and successively > > handle_userfault since we have the region registered to that. > > > > handle_userfault uses TASK_INTERRUPTIBLE, so it is interruptible by > > signals. do_user_addr_fault then busy-retries it if the pending signal > > is non-fatal. This leads to contention of the mmap_lock. > > The busy-loop causes so much contention on mmap_lock that post-copy > live migration fails to make progress, and is leading to failures. Yes? > > > This patch replaces the use of gfn_to_hva_cache with gfn_to_pfn_cache, > > as gfn_to_pfn_cache ensures page presence for the memory access, > > preventing the contention of the mmap_lock. > > > > Signed-off-by: Carsten Stollmaier > > Reviewed-by: David Woodhouse > > I think this makes sense on its own, as it addresses the specific case > where KVM is *likely* to be touching a userfaulted (guest) page. And it > allows us to ditch yet another explicit asm exception handler. At the cost of using a gpc, which has its own complexities. But I don't understand why steal_time is special. If the issue is essentially with handle_userfault(), can't this happen on any KVM uaccess?