Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode
From: Axel Rasmussen
Date: Thu Feb 25 2021 - 12:50:49 EST
On Wed, Feb 24, 2021 at 4:26 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
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On 2/18/21 4:48 PM, Axel Rasmussen wrote:
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<snip>
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> @@ -401,8 +398,10 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
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> BUG_ON(ctx->mm != mm);
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> - VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
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> - VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
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> + /* Any unrecognized flag is a bug. */
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> + VM_BUG_ON(reason & ~__VM_UFFD_FLAGS);
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> + /* 0 or > 1 flags set is a bug; we expect exactly 1. */
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> + VM_BUG_ON(!reason || !!(reason & (reason - 1)));
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I may be confused, but that seems to be checking for a flag value of 1
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as opposed to one flag being set?
(Assuming I implemented it correctly!) It's the logical negation of
this trick:
https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2
So, it's "VM_BUG_ON(reason is *not* a power of 2)".
Maybe the double negation makes it overly confusing? It ought to be
equivalent if we remove it and just say:
VM_BUG_ON(!reason || (reason & (reason - 1)));
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> if (ctx->features & UFFD_FEATURE_SIGBUS)
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> goto out;
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<snip>
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> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
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> index 3bfba75f6cbd..0388107da4b1 100644
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> --- a/mm/hugetlb.c
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> +++ b/mm/hugetlb.c
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> @@ -4352,6 +4352,38 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
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> VM_FAULT_SET_HINDEX(hstate_index(h));
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> goto backout_unlocked;
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> }
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> +
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> + /* Check for page in userfault range. */
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> + if (userfaultfd_minor(vma)) {
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> + u32 hash;
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> + struct vm_fault vmf = {
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> + .vma = vma,
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> + .address = haddr,
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> + .flags = flags,
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> + /*
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> + * Hard to debug if it ends up being used by a
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> + * callee that assumes something about the
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> + * other uninitialized fields... same as in
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> + * memory.c
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> + */
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> + };
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> +
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> + unlock_page(page);
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> +
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> + /*
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> + * hugetlb_fault_mutex and i_mmap_rwsem must be dropped
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> + * before handling userfault. Reacquire after handling
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> + * fault to make calling code simpler.
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> + */
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> +
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> + hash = hugetlb_fault_mutex_hash(mapping, idx);
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> + mutex_unlock(&hugetlb_fault_mutex_table[hash]);
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> + i_mmap_unlock_read(mapping);
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> + ret = handle_userfault(&vmf, VM_UFFD_MINOR);
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> + i_mmap_lock_read(mapping);
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> + mutex_lock(&hugetlb_fault_mutex_table[hash]);
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> + goto out;
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> + }
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> }
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> /*
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I'm good with the hugetlb.c changes. Since this in nearly identical to
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the other handle_userfault() in this routine, it might be good to create
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a common wrapper. But, that is not required.
Makes sense, I can send a v9 with a helper for this defined. I'll wait
until at least next week to do so, to pick up any other comments v8
may get in the meantime.
Thanks!
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--
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Mike Kravetz