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SubjectRe: [PATCH v17 11/23] mm: pagewalk: Add p4d_entry() and pgd_entry()
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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@arm.com>

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@vmware.com>

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


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