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ANDROID: mm: skip pte_alloc during speculative page fault

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Speculative page fault checks pmd to be valid before starting to handle
the page fault and pte_alloc() should do nothing if pmd stays valid.
If pmd gets changed during speculative page fault, we will detect the
change later and retry with mmap_lock. Therefore pte_alloc() can be
safely skipped and this prevents the racy pmd_lock() call which can
access pmd->ptl after pmd was cleared.

Bug: 257443051
Change-Id: Iec57df5530dba6e0e0bdf9f7500f910851c3d3fd
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
This commit is contained in:
Suren Baghdasaryan
2022-11-18 15:05:48 -08:00
parent d84fac9795
commit 1169f70f8f
1 changed files with 5 additions and 0 deletions
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5
mm/memory.c
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@@ -3858,6 +3858,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
if (vmf->vma_flags & VM_SHARED)
return VM_FAULT_SIGBUS;
/* Do not check unstable pmd, if it's changed will retry later */
if (vmf->flags & FAULT_FLAG_SPECULATIVE)
goto skip_pmd_checks;
/*
* Use pte_alloc() instead of pte_alloc_map(). We can't run
* pte_offset_map() on pmds where a huge pmd might be created
@@ -3875,6 +3879,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
if (unlikely(pmd_trans_unstable(vmf->pmd)))
return 0;
skip_pmd_checks:
/* Use the zero-page for reads */
if (!(vmf->flags & FAULT_FLAG_WRITE) &&
!mm_forbids_zeropage(vma->vm_mm)) {