Re: [PATCH v17 11/23] mm: pagewalk: Add p4d_entry() and pgd_entry()
From: Thomas HellstrÃm (VMware)
Date: Thu Dec 19 2019 - 09:25:49 EST
Hi, Steven,
On 12/18/19 5:23 PM, Steven Price wrote:
pgd_entry() and pud_entry() were removed by commit 0b1fbfe50006c410
("mm/pagewalk: remove pgd_entry() and pud_entry()") because there were
no users. We're about to add users so reintroduce them, along with
p4d_entry() as we now have 5 levels of tables.
Note that commit a00cc7d9dd93d66a ("mm, x86: add support for
PUD-sized transparent hugepages") already re-added pud_entry() but with
different semantics to the other callbacks. This commit reverts the
semantics back to match the other callbacks.
To support hmm.c which now uses the new semantics of pud_entry() a new
member ('action') of struct mm_walk is added which allows the callbacks
to either descend (ACTION_SUBTREE, the default), skip (ACTION_CONTINUE)
or repeat the callback (ACTION_AGAIN). hmm.c is then updated to call
pud_trans_huge_lock() itself and make use of the splitting/retry logic
of the core code.
After this change pud_entry() is called for all entries, not just
transparent huge pages.
Signed-off-by: Steven Price <steven.price@xxxxxxx>
I have a couple of comments that are actually mostly related to
pre-existing bugs, but that affect code that are touched by this patch.
Perhaps this is most cleanly addressed by a follow-up patch. Up to you
to decide.
Since these problems are pre-existing,
Reviewed-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
Thanks,
Thomas
---
include/linux/pagewalk.h | 34 ++++++++++++++++++++-----
mm/hmm.c | 55 ++++++++++++++++++++++------------------
mm/pagewalk.c | 50 +++++++++++++++++++++++++-----------
3 files changed, 94 insertions(+), 45 deletions(-)
diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 6ec82e92c87f..aa6a0b63964e 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -8,15 +8,15 @@ struct mm_walk;
/**
* mm_walk_ops - callbacks for walk_page_range
- * @pud_entry: if set, called for each non-empty PUD (2nd-level) entry
- * this handler should only handle pud_trans_huge() puds.
- * the pmd_entry or pte_entry callbacks will be used for
- * regular PUDs.
- * @pmd_entry: if set, called for each non-empty PMD (3rd-level) entry
+ * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
+ * @p4d_entry: if set, called for each non-empty P4D entry
+ * @pud_entry: if set, called for each non-empty PUD entry
+ * @pmd_entry: if set, called for each non-empty PMD entry
* this handler is required to be able to handle
* pmd_trans_huge() pmds. They may simply choose to
* split_huge_page() instead of handling it explicitly.
- * @pte_entry: if set, called for each non-empty PTE (4th-level) entry
+ * @pte_entry: if set, called for each non-empty PTE (lowest-level)
+ * entry
* @pte_hole: if set, called for each hole at all levels
* @hugetlb_entry: if set, called for each hugetlb entry
* @test_walk: caller specific callback function to determine whether
@@ -27,8 +27,15 @@ struct mm_walk;
* @pre_vma: if set, called before starting walk on a non-null vma.
* @post_vma: if set, called after a walk on a non-null vma, provided
* that @pre_vma and the vma walk succeeded.
+ *
+ * p?d_entry callbacks are called even if those levels are folded on a
+ * particular architecture/configuration.
*/
struct mm_walk_ops {
+ int (*pgd_entry)(pgd_t *pgd, unsigned long addr,
+ unsigned long next, struct mm_walk *walk);
+ int (*p4d_entry)(p4d_t *p4d, unsigned long addr,
+ unsigned long next, struct mm_walk *walk);
int (*pud_entry)(pud_t *pud, unsigned long addr,
unsigned long next, struct mm_walk *walk);
int (*pmd_entry)(pmd_t *pmd, unsigned long addr,
@@ -47,11 +54,25 @@ struct mm_walk_ops {
void (*post_vma)(struct mm_walk *walk);
};
+/*
+ * Action for pud_entry / pmd_entry callbacks.
+ * ACTION_SUBTREE is the default
+ */
+enum page_walk_action {
+ /* Descend to next level, splitting huge pages if needed and possible */
+ ACTION_SUBTREE = 0,
+ /* Continue to next entry at this level (ignoring any subtree) */
+ ACTION_CONTINUE = 1,
+ /* Call again for this entry */
+ ACTION_AGAIN = 2
+};
+
/**
* mm_walk - walk_page_range data
* @ops: operation to call during the walk
* @mm: mm_struct representing the target process of page table walk
* @vma: vma currently walked (NULL if walking outside vmas)
+ * @action: next action to perform (see enum page_walk_action)
* @private: private data for callbacks' usage
*
* (see the comment on walk_page_range() for more details)
@@ -60,6 +81,7 @@ struct mm_walk {
const struct mm_walk_ops *ops;
struct mm_struct *mm;
struct vm_area_struct *vma;
+ enum page_walk_action action;
void *private;
};
diff --git a/mm/hmm.c b/mm/hmm.c
index d379cb6496ae..05241c82e05c 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -477,20 +477,30 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
unsigned long addr = start, next;
pmd_t *pmdp;
pud_t pud;
- int ret;
+ int ret = 0;
+ spinlock_t *ptl = pud_trans_huge_lock(pudp, walk->vma);
+
+ if (!ptl)
+ return 0;
Since if we didn't get the PTL here, the pud might be unstable in which
case we want to retry:
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;
-again:
pud = READ_ONCE(*pudp);
We have the lock now, so READ_ONCE could probably be a simple dereference.
- if (pud_none(pud))
- return hmm_vma_walk_hole(start, end, walk);
+ if (pud_none(pud)) {
+ ret = hmm_vma_walk_hole(start, end, walk);
+ goto out_unlock;
+ }
pud_trans_huge_lock() successful return means pud_none() is always false.
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))
- return hmm_vma_walk_hole(start, end, walk);
+ if (!pud_present(pud)) {
+ ret = hmm_vma_walk_hole(start, end, walk);
+ goto out_unlock;
+ }
Can't see !pud_present happening either after a succesful
pud_trans_huge_lock().
i = (addr - range->start) >> PAGE_SHIFT;
npages = (end - addr) >> PAGE_SHIFT;
@@ -499,16 +509,20 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
cpu_flags = pud_to_hmm_pfn_flags(range, pud);
hmm_range_need_fault(hmm_vma_walk, pfns, npages,
cpu_flags, &fault, &write_fault);
- if (fault || write_fault)
- return hmm_vma_walk_hole_(addr, end, fault,
- write_fault, walk);
+ if (fault || write_fault) {
+ ret = hmm_vma_walk_hole_(addr, end, fault,
+ write_fault, walk);
+ goto out_unlock;
+ }
pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
for (i = 0; i < npages; ++i, ++pfn) {
hmm_vma_walk->pgmap = get_dev_pagemap(pfn,
hmm_vma_walk->pgmap);
- if (unlikely(!hmm_vma_walk->pgmap))
- return -EBUSY;
+ if (unlikely(!hmm_vma_walk->pgmap)) {
+ ret = -EBUSY;
+ goto out_unlock;
+ }
pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
cpu_flags;
}
@@ -517,22 +531,15 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
hmm_vma_walk->pgmap = NULL;
}
hmm_vma_walk->last = end;
- return 0;
+ goto out_unlock;
}
- split_huge_pud(walk->vma, pudp, addr);
- if (pud_none(*pudp))
- goto again;
+ /* Ask for the PUD to be split */
+ walk->action = ACTION_SUBTREE;
- pmdp = pmd_offset(pudp, addr);
- do {
- next = pmd_addr_end(addr, end);
- ret = hmm_vma_walk_pmd(pmdp, addr, next, walk);
- if (ret)
- return ret;
- } while (pmdp++, addr = next, addr != end);
-
- return 0;
+out_unlock:
+ spin_unlock(ptl);
+ return ret;
}
#else
#define hmm_vma_walk_pud NULL
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index ea0b9e606ad1..690af44609e2 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -46,6 +46,9 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
break;
continue;
}
+
+ walk->action = ACTION_SUBTREE;
+
/*
* This implies that each ->pmd_entry() handler
* needs to know about pmd_trans_huge() pmds
@@ -55,16 +58,21 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
if (err)
break;
+ if (walk->action == ACTION_AGAIN)
+ goto again;
+
/*
* Check this here so we only break down trans_huge
* pages when we _need_ to
*/
- if (!ops->pte_entry)
+ if (walk->action == ACTION_CONTINUE ||
+ !(ops->pte_entry))
continue;
split_huge_pmd(walk->vma, pmd, addr);
if (pmd_trans_unstable(pmd))
goto again;
+
err = walk_pte_range(pmd, addr, next, walk);
if (err)
break;
@@ -93,24 +101,25 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
continue;
}
- if (ops->pud_entry) {
- spinlock_t *ptl = pud_trans_huge_lock(pud, walk->vma);
+ walk->action = ACTION_SUBTREE;
- if (ptl) {
- err = ops->pud_entry(pud, addr, next, walk);
- spin_unlock(ptl);
- if (err)
- break;
- continue;
- }
- }
+ if (ops->pud_entry)
+ err = ops->pud_entry(pud, addr, next, walk);
+ if (err)
+ break;
+
+ if (walk->action == ACTION_AGAIN)
+ goto again;
+
+ if (walk->action == ACTION_CONTINUE ||
+ !(ops->pmd_entry || ops->pte_entry))
+ continue;
split_huge_pud(walk->vma, pud, addr);
if (pud_none(*pud))
Here we should really have
if (pud_trans_unstable(pud))
Thanks,
Thomas