On 19/12/2019 14:25, Thomas Hellström (VMware) wrote:
> 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, I'll send out an extra patch with these changes.
Steve
> 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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