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mm: handle shared faults under the VMA lock

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There are many implementations of ->fault and some of them depend on
mmap_lock being held.  All vm_ops that implement ->map_pages() end up
calling filemap_fault(), which I have audited to be sure it does not rely
on mmap_lock.  So (for now) key off ->map_pages existing as a flag to
indicate that it's safe to call ->fault while only holding the vma lock.

Link: https://lkml.kernel.org/r/20231006195318.4087158-4-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2023-10-06 20:53:15 +01:00 committed by Andrew Morton
parent 164b06f238
commit 4ed4379881
1 changed files with 18 additions and 4 deletions
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22
mm/memory.c
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@ -3042,6 +3042,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
count_vm_event(PGREUSE);
}
/*
* We could add a bitflag somewhere, but for now, we know that all
* vm_ops that have a ->map_pages have been audited and don't need
* the mmap_lock to be held.
*/
static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
if (vma->vm_ops->map_pages || !(vmf->flags & FAULT_FLAG_VMA_LOCK))
return 0;
vma_end_read(vma);
return VM_FAULT_RETRY;
}
static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
@ -4669,10 +4684,9 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
vm_fault_t ret, tmp;
struct folio *folio;
if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
vma_end_read(vma);
return VM_FAULT_RETRY;
}
ret = vmf_can_call_fault(vmf);
if (ret)
return ret;
ret = __do_fault(vmf);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))