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mm: check against orig_pte for finish_fault() 

This patch allows do_fault() to trigger on !pte_none() cases too.  This
prepares for the pte markers to be handled by do_fault() just like none
pte.

To achieve this, instead of unconditionally check against pte_none() in
finish_fault(), we may hit the case that the orig_pte was some pte marker
so what we want to do is to replace the pte marker with some valid pte
entry.  Then if orig_pte was set we'd want to check the current *pte
(under pgtable lock) against orig_pte rather than none pte.

Right now there's no solid way to safely reference orig_pte because when
pmd is not allocated handle_pte_fault() will not initialize orig_pte, so
it's not safe to reference it.

There's another solution proposed before this patch to do pte_clear() for
vmf->orig_pte for pmd==NULL case, however it turns out it'll break arm32
because arm32 could have assumption that pte_t* pointer will always reside
on a real ram32 pgtable, not any kernel stack variable.

To solve this, we add a new flag FAULT_FLAG_ORIG_PTE_VALID, and it'll be
set along with orig_pte when there is valid orig_pte, or it'll be cleared
when orig_pte was not initialized.

It'll be updated every time we call handle_pte_fault(), so e.g.  if a page
fault retry happened it'll be properly updated along with orig_pte.

[1] https://lore.kernel.org/lkml/710c48c9-406d-e4c5-a394-10501b951316@samsung.com/

[akpm@linux-foundation.org: coding-style cleanups]
[peterx@redhat.com: fix crash reported by Marek]
  Link: https://lkml.kernel.org/r/Ylb9rXJyPm8/ao8f@xz-m1.local
Link: https://lkml.kernel.org/r/20220405014836.14077-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Peter Xu 2022-05-12 20:22:52 -07:00 committed by Andrew Morton
parent 5c041f5d1f
commit f46f2adecd
2 changed files with 14 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/mm_types.h
View File

@ -822,6 +822,8 @@ typedef struct {
* @FAULT_FLAG_UNSHARE: The fault is an unsharing request to unshare (and mark
* exclusive) a possibly shared anonymous page that is
* mapped R/O.
* @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
* We should only access orig_pte if this flag set.
*
* About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
* whether we would allow page faults to retry by specifying these two
@ -858,6 +860,7 @@ enum fault_flag {
FAULT_FLAG_INSTRUCTION = 1 << 8,
FAULT_FLAG_INTERRUPTIBLE = 1 << 9,
FAULT_FLAG_UNSHARE = 1 << 10,
FAULT_FLAG_ORIG_PTE_VALID = 1 << 11,
};
#endif /* _LINUX_MM_TYPES_H */

12
mm/memory.c
View File

@ -4183,6 +4183,14 @@ void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
set_pte_at(vma->vm_mm, addr, vmf->pte, entry);
}
static bool vmf_pte_changed(struct vm_fault *vmf)
{
if (vmf->flags & FAULT_FLAG_ORIG_PTE_VALID)
return !pte_same(*vmf->pte, vmf->orig_pte);
return !pte_none(*vmf->pte);
}
/**
* finish_fault - finish page fault once we have prepared the page to fault
*
@ -4241,7 +4249,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
vmf->address, &vmf->ptl);
ret = 0;
/* Re-check under ptl */
if (likely(pte_none(*vmf->pte)))
if (likely(!vmf_pte_changed(vmf)))
do_set_pte(vmf, page, vmf->address);
else
ret = VM_FAULT_NOPAGE;
@ -4709,6 +4717,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
* concurrent faults and from rmap lookups.
*/
vmf->pte = NULL;
vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
} else {
/*
* If a huge pmd materialized under us just retry later. Use
@ -4732,6 +4741,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
*/
vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
vmf->orig_pte = *vmf->pte;
vmf->flags |= FAULT_FLAG_ORIG_PTE_VALID;
/*
* some architectures can have larger ptes than wordsize,