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b=pBile50QnlGw8x75Ig9U45/zvrTbeGA0ov5UjecXOYj8eYIHEOhIX+dMpbkvVuR0W KaxG0ErlZlPaC9kxvgycPVLALXmhDSQoT1+nQjyuzLDUT5H8vJmsc62X6Y0T9uQaHR 4Qo3+b9PwRuH9v2n3d6JoxuwZYR/0fN/oL8ep4/Q= Date: Fri, 30 Jan 2026 12:38:10 -0800 From: Andrew Morton To: Thomas =?ISO-8859-1?Q?Hellstr=F6m?= Cc: intel-xe@lists.freedesktop.org, Ralph Campbell , Christoph Hellwig , Jason Gunthorpe , Jason Gunthorpe , Leon Romanovsky , Matthew Brost , linux-mm@kvack.org, stable@vger.kernel.org, dri-devel@lists.freedesktop.org Subject: Re: [PATCH] mm/hmm: Fix a hmm_range_fault() livelock / starvation problem Message-Id: <20260130123810.61dde600422a8fe01cff8296@linux-foundation.org> In-Reply-To: References: <20260130144529.79909-1-thomas.hellstrom@linux.intel.com> <20260130100013.fb1ce1cd5bd7a440087c7b37@linux-foundation.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam12 X-Stat-Signature: 318oixjkinbou5dj75u6efucot77wodt X-Rspamd-Queue-Id: 36911140004 X-Rspam-User: X-HE-Tag: 1769805492-460008 X-HE-Meta: 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 5P2LTba6 TS1SAxgLTDUomDhVKA/98RVX/rnGtedUUPmtFMTeUm/FZJzdoDgQVamHEj0as8TDtawXlEehdJA/o5H20PyYYHbLyM9ZxkJaILATAcjpnxDLufsmOl7u4HK27X6qseCA9TEe28M1Qz6MRKPsOhfoQe+U4AiUXMXnUl3U0NE5lJ3reAPtsDmxMs3jdkIvZUVw575/8UoRuTAEoIWCgM5o1EZQkUgRTPZt4k8JQf9BvbSOV7f6q6/zDkzZryb1ETS1yHYUcbkU/Gl3+rtU017Nov6Z04ROO1pmZBkD0uGno0XPOo1Cle2kt9+lhZF2EMVbwAZFf 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, 30 Jan 2026 20:56:31 +0100 Thomas Hellstr=F6m wrote: > >=20 > > > --- a/mm/hmm.c > > > +++ b/mm/hmm.c > > > @@ -674,6 +674,13 @@ int hmm_range_fault(struct hmm_range *range) > > > =A0 return -EBUSY; > > > =A0 ret =3D walk_page_range(mm, hmm_vma_walk.last, > > > range->end, > > > =A0 =A0=A0=A0=A0=A0 &hmm_walk_ops, > > > &hmm_vma_walk); > > > + /* > > > + * Conditionally reschedule to let other work > > > items get > > > + * a chance to unlock device-private pages whose > > > locks > > > + * we're spinning on. > > > + */ > > > + cond_resched(); > > > + > > > =A0 /* > > > =A0 * When -EBUSY is returned the loop restarts with > > > =A0 * hmm_vma_walk.last set to an address that has > > > not been stored > >=20 > > If the process which is running hmm_range_fault() has > > SCHED_FIFO/SHCED_RR then cond_resched() doesn't work.=A0 An explicit > > msleep() would be better? >=20 > Unfortunately hmm_range_fault() is typically called from a gpu > pagefault handler and it's crucial to get the gpu up and running again > as fast as possible. Would a millisecond matter? Regular old preemption will often cause longer delays. > Is there a way we could test for the cases where cond_resched() doesn't > work and in that case instead call sched_yield(), at least on -EBUSY > errors? kernel-internal sched_yield() was taken away years ago and I don't think there's a replacement, particularly one which will cause a realtime-policy task to yield to a non-rt-policy one. It's common for kernel code to forget that it could have realtime policy - we probably have potential lockups in various places. I suggest you rerun your testcase with this patch using `chrt -r', see if my speculation is correct.