5.15-stable review patch. If anyone has any objections, please let me know.
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From: Claudio Imbrenda imbrenda@linux.ibm.com
[ Upstream commit f0a1a0615a6ff6d38af2c65a522698fb4bb85df6 ]
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).
Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests") Signed-off-by: Claudio Imbrenda imbrenda@linux.ibm.com Reviewed-by: Janosch Frank frankja@linux.ibm.com Reviewed-by: Christian Borntraeger borntraeger@de.ibm.com Link: https://lore.kernel.org/r/20210920132502.36111-5-imbrenda@linux.ibm.com Signed-off-by: Christian Borntraeger borntraeger@de.ibm.com Stable-dep-of: 3f29f6537f54 ("s390/uv: Don't call folio_wait_writeback() without a folio reference") Signed-off-by: Sasha Levin sashal@kernel.org --- arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------ arch/s390/kvm/intercept.c | 5 +++++ 2 files changed, 28 insertions(+), 6 deletions(-)
diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c index f95ccbd396925..09b80d371409b 100644 --- a/arch/s390/kernel/uv.c +++ b/arch/s390/kernel/uv.c @@ -165,7 +165,7 @@ 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; @@ -181,12 +181,25 @@ 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; }
/* @@ -239,6 +252,10 @@ 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) { /* diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c index 458b42b50b8cb..79e6f016c22fc 100644 --- a/arch/s390/kvm/intercept.c +++ b/arch/s390/kvm/intercept.c @@ -537,6 +537,11 @@ static int handle_pv_uvc(struct kvm_vcpu *vcpu) */ if (rc == -EINVAL) return 0; + /* + * If we got -EAGAIN here, we simply return it. It will eventually + * get propagated all the way to userspace, which should then try + * again. + */ return rc; }