On 22.08.24 09:13, Qi Zheng wrote:
Currently, the usage of pte_offset_map_nolock() can be divided into the
following two cases:
1) After acquiring PTL, only read-only operations are performed on the PTE
page. In this case, the RCU lock in pte_offset_map_nolock() will ensure
that the PTE page will not be freed, and there is no need to worry
about whether the pmd entry is modified.
There is also the usage where we don't grab the PTL at all, and only do
a racy (read-only) lookup.
2) After acquiring PTL, the pte or pmd entries may be modified. At this
time, we need to ensure that the pmd entry has not been modified
concurrently.
To more clearing distinguish between these two cases, this commit
introduces two new helper functions to replace pte_offset_map_nolock().
For 1), just rename it to pte_offset_map_ro_nolock(). For 2), in addition
to changing the name to pte_offset_map_rw_nolock(), it also outputs the
pmdval when successful. This can help the caller recheck *pmd once the PTL
is taken. In some cases, that is, either the mmap_lock for write, or
pte_same() check on contents, is also enough to ensure that the pmd entry
is stable. But in order to prevent the interface from being abused, we
choose to pass in a dummy local variable instead of NULL.
Subsequent commits will convert pte_offset_map_nolock() into the above
two functions one by one, and finally completely delete it.
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
Documentation/mm/split_page_table_lock.rst | 7 ++++
include/linux/mm.h | 5 +++
mm/pgtable-generic.c | 43 ++++++++++++++++++++++
3 files changed, 55 insertions(+)
diff --git a/Documentation/mm/split_page_table_lock.rst b/Documentation/mm/split_page_table_lock.rst
index e4f6972eb6c04..08d0e706a32db 100644
--- a/Documentation/mm/split_page_table_lock.rst
+++ b/Documentation/mm/split_page_table_lock.rst
@@ -19,6 +19,13 @@ There are helpers to lock/unlock a table and other accessor functions:
- pte_offset_map_nolock()
maps PTE, returns pointer to PTE with pointer to its PTE table
lock (not taken), or returns NULL if no PTE table;
What will happen to pte_offset_map_nolock() after this series? Does it
still exist or will it become an internal helper?
+ - pte_offset_map_ro_nolock()
+ maps PTE, returns pointer to PTE with pointer to its PTE table
+ lock (not taken), or returns NULL if no PTE table;
+ - pte_offset_map_rw_nolock()
+ maps PTE, returns pointer to PTE with pointer to its PTE table
+ lock (not taken) and the value of its pmd entry, or returns NULL
+ if no PTE table;
[...]
+pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
+ unsigned long addr, pmd_t *pmdvalp,
+ spinlock_t **ptlp)
+{
+ pmd_t pmdval;
+ pte_t *pte;
+
+ BUG_ON(!pmdvalp);
As raised, no BUG_ON please. VM_WARN_ON_ONCE() is helpful during early
testing and should catch these kind of things.
If someone thinks not requiring a non-NULL pointer is better, please
speak up, I'm not married to that idea :)
+ pte = __pte_offset_map(pmd, addr, &pmdval);
+ if (likely(pte))
+ *ptlp = pte_lockptr(mm, &pmdval);
+ *pmdvalp = pmdval;
+ return pte;
+}
+
/*
* pte_offset_map_lock(mm, pmd, addr, ptlp), and its internal implementation
* __pte_offset_map_lock() below, is usually called with the pmd pointer for
@@ -356,6 +383,22 @@ pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
* recheck *pmd once the lock is taken; in practice, no callsite needs that -
* either the mmap_lock for write, or pte_same() check on contents, is enough.
*
+ * pte_offset_map_ro_nolock(mm, pmd, addr, ptlp), above, is like
+ * pte_offset_map(); but when successful, it also outputs a pointer to the
+ * spinlock in ptlp - as pte_offset_map_lock() does, but in this case without
+ * locking it. This helps the caller to avoid a later pte_lockptr(mm, *pmd),
+ * which might by that time act on a changed *pmd: pte_offset_map_ro_nolock()
+ * provides the correct spinlock pointer for the page table that it returns.
+ * For readonly case, the caller does not need to recheck *pmd after the lock is
+ * taken, because the RCU lock will ensure that the PTE page will not be freed. > + *
+ * pte_offset_map_rw_nolock(mm, pmd, addr, pmdvalp, ptlp), above, is like
+ * pte_offset_map_ro_nolock(); but when successful, it also outputs the
+ * pdmval. For cases where pte or pmd entries may be modified, that is, maywrite
+ * case, this can help the caller recheck *pmd once the lock is taken. In some
+ * cases, that is, either the mmap_lock for write, or pte_same() check on
+ * contents, is also enough to ensure that the pmd entry is stable.
+ *
* Note that free_pgtables(), used after unmapping detached vmas, or when
* exiting the whole mm, does not take page table lock before freeing a page
* table, and may not use RCU at all: "outsiders" like khugepaged should avoid
In general to me a step into the right direction. Likely the
documentation could be further clarified in some aspects:
Like that the use of pte_offset_map_ro_nolock() does not allow to easily
identify if the page table was replaced in the meantime. Even after
grabbing the PTL, we might be looking either at a page table that is
still mapped or one that was unmapped and is about to get freed. But for
R/O access this is usually sufficient AFAIUK.
Or that "RO" / "RW" expresses the intended semantics, not that the
*kmap* will be RO/RW protected.
--
Cheers,
David / dhildenb
