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Subject[PATCH] mm/hmm: Cleanup hmm_vma_walk_pud()/walk_pud_range()
Date
There are a number of minor misuses of the page table APIs in
hmm_vma_walk_pud():

If the pud_trans_huge_lock() hasn't been obtained it might be because
the PUD is unstable, so we should retry.

If it has been obtained then there's no need for a READ_ONCE, and the
PUD cannot be pud_none() or !pud_present() so these paths are dead code.

Finally in walk_pud_range(), after a call to split_huge_pud() the code
should check pud_trans_unstable() rather than pud_none() to decide
whether the PUD should be retried.

Suggested-by: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Signed-off-by: Steven Price <steven.price@arm.com>
---
This is based on top of my "Generic page walk and ptdump" series and
fixes some pre-existing bugs spotted by Thomas.

mm/hmm.c | 16 +++++-----------
mm/pagewalk.c | 2 +-
2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index a71295e99968..d4aae4dcc6e8 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -480,28 +480,22 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
int ret = 0;
spinlock_t *ptl = pud_trans_huge_lock(pudp, walk->vma);

- if (!ptl)
+ if (!ptl) {
+ if (pud_trans_unstable(pudp))
+ walk->action = ACTION_AGAIN;
return 0;
+ }

/* Normally we don't want to split the huge page */
walk->action = ACTION_CONTINUE;

- pud = READ_ONCE(*pudp);
- if (pud_none(pud)) {
- ret = hmm_vma_walk_hole(start, end, -1, walk);
- goto out_unlock;
- }
+ pud = *pudp;

if (pud_huge(pud) && pud_devmap(pud)) {
unsigned long i, npages, pfn;
uint64_t *pfns, cpu_flags;
bool fault, write_fault;

- if (!pud_present(pud)) {
- ret = hmm_vma_walk_hole(start, end, -1, walk);
- goto out_unlock;
- }
-
i = (addr - range->start) >> PAGE_SHIFT;
npages = (end - addr) >> PAGE_SHIFT;
pfns = &range->pfns[i];
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 5895ce4f1a85..4598f545b869 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -154,7 +154,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,

if (walk->vma)
split_huge_pud(walk->vma, pud, addr);
- if (pud_none(*pud))
+ if (pud_trans_unstable(pud))
goto again;

err = walk_pmd_range(pud, addr, next, walk);
--
2.20.1
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