Re: [PATCH 03/10] mm/hugetlb: Document huge_pte_offset usage
From: David Hildenbrand
Date: Wed Nov 30 2022 - 11:33:03 EST
On 30.11.22 17:25, Peter Xu wrote:
On Wed, Nov 30, 2022 at 05:11:36PM +0100, David Hildenbrand wrote:
On 30.11.22 17:09, Peter Xu wrote:
On Wed, Nov 30, 2022 at 11:24:34AM +0100, David Hildenbrand wrote:
On 29.11.22 20:35, Peter Xu wrote:
huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a
hugetlb address.
Normally, it's always safe to walk a generic pgtable as long as we're with
the mmap lock held for either read or write, because that guarantees the
pgtable pages will always be valid during the process.
But it's not true for hugetlbfs, especially shared: hugetlbfs can have its
pgtable freed by pmd unsharing, it means that even with mmap lock held for
current mm, the PMD pgtable page can still go away from under us if pmd
unsharing is possible during the walk.
So we have two ways to make it safe even for a shared mapping:
(1) If we're with the hugetlb vma lock held for either read/write, it's
okay because pmd unshare cannot happen at all.
(2) If we're with the i_mmap_rwsem lock held for either read/write, it's
okay because even if pmd unshare can happen, the pgtable page cannot
be freed from under us.
Document it.
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
include/linux/hugetlb.h | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 551834cd5299..81efd9b9baa2 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -192,6 +192,38 @@ extern struct list_head huge_boot_pages;
pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long addr, unsigned long sz);
+/*
+ * huge_pte_offset(): Walk the hugetlb pgtable until the last level PTE.
+ * Returns the pte_t* if found, or NULL if the address is not mapped.
+ *
+ * Since this function will walk all the pgtable pages (including not only
+ * high-level pgtable page, but also PUD entry that can be unshared
+ * concurrently for VM_SHARED), the caller of this function should be
+ * responsible of its thread safety. One can follow this rule:
+ *
+ * (1) For private mappings: pmd unsharing is not possible, so it'll
+ * always be safe if we're with the mmap sem for either read or write.
+ * This is normally always the case, IOW we don't need to do anything
+ * special.
Maybe worth mentioning that hugetlb_vma_lock_read() and friends already
optimize for private mappings, to not take the VMA lock if not required.
Yes we can. I assume this is not super urgent so I'll hold a while to see
whether there's anything else that needs amending for the documents.
Btw, even with hugetlb_vma_lock_read() checking SHARED for a private only
code path it's still better to not take the lock at all, because that still
contains a function jump which will be unnecesary.
IMHO it makes coding a lot more consistent and less error-prone when not
care about whether to the the lock or not (as an optimization) and just
having this handled "automatically".
Optimizing a jump out would rather smell like a micro-optimization.
Or we can move the lock helpers into the headers, too.
Ah, yes.
--
Thanks,
David / dhildenb