From e709accc7670a07b2e5186449f0640c2416662ec Mon Sep 17 00:00:00 2001 From: Jason Gunthorpe Date: Wed, 24 Jul 2019 08:52:58 +0200 Subject: [PATCH] mm/hmm: comment on VM_FAULT_RETRY semantics in handle_mm_fault The magic dropping of mmap_sem when handle_mm_fault returns VM_FAULT_RETRY is rather subtile. Add a comment explaining it. Link: https://lore.kernel.org/r/20190724065258.16603-8-hch@lst.de Tested-by: Ralph Campbell Signed-off-by: Jason Gunthorpe [hch: wrote a changelog] Signed-off-by: Christoph Hellwig --- mm/hmm.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/mm/hmm.c b/mm/hmm.c index 16b6731a34db..54b3a4162ae9 100644 --- a/mm/hmm.c +++ b/mm/hmm.c @@ -301,8 +301,10 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr, flags |= hmm_vma_walk->block ? 0 : FAULT_FLAG_ALLOW_RETRY; flags |= write_fault ? FAULT_FLAG_WRITE : 0; ret = handle_mm_fault(vma, addr, flags); - if (ret & VM_FAULT_RETRY) + if (ret & VM_FAULT_RETRY) { + /* Note, handle_mm_fault did up_read(&mm->mmap_sem)) */ return -EAGAIN; + } if (ret & VM_FAULT_ERROR) { *pfn = range->values[HMM_PFN_ERROR]; return -EFAULT;