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Tue, 31 Aug 2021 14:32:25 +0000 (GMT) Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 03FC9AE05F; Tue, 31 Aug 2021 14:32:25 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.145.164.122]) by d06av26.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 31 Aug 2021 14:32:24 +0000 (GMT) Subject: Re: [PATCH v4 04/14] KVM: s390: pv: avoid stalls when making pages secure To: Claudio Imbrenda , kvm@vger.kernel.org Cc: cohuck@redhat.com, frankja@linux.ibm.com, thuth@redhat.com, pasic@linux.ibm.com, david@redhat.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, Ulrich.Weigand@de.ibm.com References: <20210818132620.46770-1-imbrenda@linux.ibm.com> <20210818132620.46770-5-imbrenda@linux.ibm.com> From: Christian Borntraeger Message-ID: <731aeb25-3883-5941-9400-7cd8c43fc31c@de.ibm.com> Date: Tue, 31 Aug 2021 16:32:24 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20210818132620.46770-5-imbrenda@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: nmgsrCJs4maGmaCDAAGwWqGZKiYANIHd X-Proofpoint-ORIG-GUID: 4sx5EiE5QIMtZANOyzQ3koe6epN6LH9T X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.790 definitions=2021-08-31_05:2021-08-31,2021-08-31 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 mlxscore=0 spamscore=0 bulkscore=0 suspectscore=0 adultscore=0 phishscore=0 priorityscore=1501 malwarescore=0 clxscore=1015 mlxlogscore=999 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2107140000 definitions=main-2108310078 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18.08.21 15:26, Claudio Imbrenda wrote: > Improve make_secure_pte to avoid stalls when the system is heavily > overcommitted. This was especially problematic in kvm_s390_pv_unpack, > because of the loop over all pages that needed unpacking. > > Due to the locks being held, it was not possible to simply replace > uv_call with uv_call_sched. A more complex approach was > needed, in which uv_call is replaced with __uv_call, which does not > loop. When the UVC needs to be executed again, -EAGAIN is returned, and > the caller (or its caller) will try again. > > When -EAGAIN is returned, the path is the same as when the page is in > writeback (and the writeback check is also performed, which is > harmless). To me it looks like handle_pv_uvc does not handle EAGAIN but also calls into this code. Is this code path ok or do we need to change something here? > > Signed-off-by: Claudio Imbrenda > Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests") > --- > arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------ > 1 file changed, 23 insertions(+), 6 deletions(-) > > diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c > index aeb0a15bcbb7..68a8fbafcb9c 100644 > --- a/arch/s390/kernel/uv.c > +++ b/arch/s390/kernel/uv.c > @@ -180,7 +180,7 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr, > { > pte_t entry = READ_ONCE(*ptep); > struct page *page; > - int expected, rc = 0; > + int expected, cc = 0; > > if (!pte_present(entry)) > return -ENXIO; > @@ -196,12 +196,25 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr, > if (!page_ref_freeze(page, expected)) > return -EBUSY; > set_bit(PG_arch_1, &page->flags); > - rc = uv_call(0, (u64)uvcb); > + /* > + * If the UVC does not succeed or fail immediately, we don't want to > + * loop for long, or we might get stall notifications. > + * On the other hand, this is a complex scenario and we are holding a lot of > + * locks, so we can't easily sleep and reschedule. We try only once, > + * and if the UVC returned busy or partial completion, we return > + * -EAGAIN and we let the callers deal with it. > + */ > + cc = __uv_call(0, (u64)uvcb); > page_ref_unfreeze(page, expected); > - /* Return -ENXIO if the page was not mapped, -EINVAL otherwise */ > - if (rc) > - rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL; > - return rc; > + /* > + * Return -ENXIO if the page was not mapped, -EINVAL for other errors. > + * If busy or partially completed, return -EAGAIN. > + */ > + if (cc == UVC_CC_OK) > + return 0; > + else if (cc == UVC_CC_BUSY || cc == UVC_CC_PARTIAL) > + return -EAGAIN; > + return uvcb->rc == 0x10a ? -ENXIO : -EINVAL; > } > > /* > @@ -254,6 +267,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb) > mmap_read_unlock(gmap->mm); > > if (rc == -EAGAIN) { > + /* > + * If we are here because the UVC returned busy or partial > + * completion, this is just a useless check, but it is safe. > + */ > wait_on_page_writeback(page); > } else if (rc == -EBUSY) { > /* >